diff --git a/app/controllers/admin_controller.rb b/app/controllers/admin_controller.rb
index 36f7e7ab..c2fd8416 100644
--- a/app/controllers/admin_controller.rb
+++ b/app/controllers/admin_controller.rb
@@ -4,7 +4,7 @@ class AdminController < ApplicationController
   before_action :require_admin_user
 
   def index
-    @maps = filter_and_sort_maps(Map.includes(:layers, :user))
+    @maps = filter_and_sort_maps(Map.includes(:layers, :owners))
 
     respond_to do |format|
       format.html # full page
diff --git a/app/controllers/concerns/map_list_filters.rb b/app/controllers/concerns/map_list_filters.rb
index 4b95d709..cb838fc2 100644
--- a/app/controllers/concerns/map_list_filters.rb
+++ b/app/controllers/concerns/map_list_filters.rb
@@ -11,7 +11,10 @@ def filter_and_sort_maps(maps)
       @searchuser = User.find(userid) if userid.present?
     end
 
-    maps = maps.where(user: @searchuser) if @searchuser
+    if @searchuser
+      # Find maps where user is in owner_ids array
+      maps = maps.where(owner_ids: @searchuser.id)
+    end
     maps = maps.search(@filter) unless @filter.empty?
     maps = maps.sorted(@sort, @direction)
     maps.limit(params[:limit] || 300)
diff --git a/app/controllers/maps_controller.rb b/app/controllers/maps_controller.rb
index 44236f5d..c6912ebf 100644
--- a/app/controllers/maps_controller.rb
+++ b/app/controllers/maps_controller.rb
@@ -13,12 +13,12 @@ class MapsController < ApplicationController
   layout "map", only: [ :show, :tutorial ]
 
   def index
-    @maps = filter_and_sort_maps(Map.unscoped.listed.includes(:layers, :user))
+    @maps = filter_and_sort_maps(Map.unscoped.listed.includes(:layers, :owners))
   end
 
   def my
     @recent_map_ids = @user.recent_map_ids
-    @my_maps = filter_and_sort_maps(Map.where(user: @user).includes(:layers, :user))
+    @my_maps = filter_and_sort_maps(@user.owned_maps.includes(:layers, :owners))
 
     respond_to do |format|
       format.html # full page
@@ -41,7 +41,7 @@ def show
 
         gon.map_id = params[:id]
         gon.user_id = @user.id if @user
-        gon.edit_id = @map.private_id.to_s if @user&.admin? || (@user && @map.user == @user)
+        gon.edit_id = @map.private_id.to_s if @user&.admin? || @map.owned_by?(@user)
         gon.map_mode = @map_mode
         gon.rails_env = Rails.env
         gon.csrf_token = form_authenticity_token
@@ -70,7 +70,9 @@ def tutorial
 
   def create
     coords = ip_coordinates
-    @map = Map.create!(user: @user, center: coords)
+    @map = Map.new(center: coords)
+    @map.add_owner(@user)
+    @map.save!
 
     redirect_to @map.private_map_path, notice: "Map was successfully created."
   end
@@ -78,7 +80,8 @@ def create
   def copy
     require_map_owner if @map.view_permission == "private"
     cloned_map = @map.clone_with_layers
-    cloned_map.update(user: @user, name: "Copy of " + @map.name.to_s)
+    cloned_map.update(name: "Copy of " + @map.name.to_s)
+    cloned_map.add_owner(@user)
     redirect_to cloned_map.private_map_path, notice: "Map was successfully copied."
   end
 
@@ -136,7 +139,7 @@ def ip_coordinates
   # :nocov:
 
   def require_map_owner
-    if !(@user&.admin? || (@map.user && @map.user == @user))
+    if !(@user&.admin? || @map.owned_by?(@user))
       Rails.logger.warn "Map view requires owner permissions, but current user isn't."
       redirect_to maps_path
     end
diff --git a/app/models/map.rb b/app/models/map.rb
index a711dd1a..aeb896eb 100644
--- a/app/models/map.rb
+++ b/app/models/map.rb
@@ -5,7 +5,11 @@ class Map
   include Turbo::Broadcastable
 
   has_many :layers, dependent: :destroy
-  belongs_to :user, optional: true, counter_cache: true
+
+  # Many-to-many relationship with users (stores owner_ids array)
+  has_and_belongs_to_many :owners,
+    class_name: "User",
+    inverse_of: nil
 
   # implicit_order_column is not supported by mongoid
   default_scope { sorted(:created_at, :asc) }
@@ -34,8 +38,8 @@ class Map
   field :bearing, type: String
   field :name, type: String
   field :description, type: String
-  field :private_id, type: String, default: -> { SecureRandom.hex(6).tap { |i| i[0..1] = "11" } }
-  field :public_id, type: String, default: -> { SecureRandom.hex(4).tap { |i| i[0..1] = "11" } }
+  field :private_id, type: String, default: -> { SecureRandom.hex(6) }
+  field :public_id, type: String, default: -> { SecureRandom.hex(4) }
   field :viewed_at, type: DateTime
   field :view_count, type: Integer, default: 0
   field :type, type: String
@@ -102,6 +106,23 @@ class Map
     format: { without: /\//, message: "private_id cannot contain a '/'" },
     if: :will_save_change_to_private_id?
 
+  # Check if a user is an owner
+  def owned_by?(user)
+    return false unless user
+    owner_ids.include?(user.id)
+  end
+
+  # Add an owner (idempotent)
+  def add_owner(user)
+    return if owned_by?(user)
+    owners << user
+  end
+
+  # Remove an owner (allows maps with no owners for anonymous/demo maps)
+  def remove_owner(user)
+    owners.delete(user)
+  end
+
   def properties
     { name: name,
      description: description,
@@ -207,10 +228,11 @@ def self.tutorial_map(user)
     tutorial_file = Rails.root.join("db/seeds/demo.json")
 
     if user&.name
-      unless (map = Map.tutorial.where(user: user).first)
+      unless (map = user.owned_maps.tutorial.first)
         map = Map.create_from_file(tutorial_file)
         name = user.name.split.first
-        map.update(user: user, type: "tutorial")
+        map.update(type: "tutorial")
+        map.add_owner(user)
         map.features.where("properties.label" => "Welcome to the Mapforge Tutorial map")
           .update_all("properties.label" => "Welcome #{name} to the Mapforge Tutorial map")
       end
@@ -299,6 +321,7 @@ def delete_screenshot
     File.delete(screenshot_file) if File.exist?(screenshot_file)
   end
 
+  # make public id safe for file names
   def safe_public_id
     separator = "_"
     public_id.strip
diff --git a/app/models/user.rb b/app/models/user.rb
index 2c208cd1..62da8b3e 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -3,12 +3,10 @@ class User
   include Mongoid::Document
   include Mongoid::Timestamps
 
-  has_many :maps
-
   scope :admin, -> { where(admin: true) }
   scope :github, -> { where(provider: "github") }
   scope :google, -> { where(provider: "google_oauth2") }
-  scope :with_maps, -> { where(:maps_count.gt => 0) }
+  scope :with_maps, -> { where(:_id.in => Map.distinct(:owner_ids)) }
   scope :with_images, -> { where(:images_count.gt => 0) }
 
   field :uid
@@ -17,10 +15,14 @@ class User
   field :email
   field :image
   field :admin
-  field :maps_count, type: Integer, default: 0
   field :images_count, type: Integer, default: 0
   field :recent_map_ids, type: Array, default: []
 
+  # Get all maps where this user is an owner
+  def owned_maps
+    Map.where(owner_ids: id)
+  end
+
   # Track a map view and maintain a limited history of recently viewed maps
   # The list can store private as well as public map ids
   # @param map [Map] the map being viewed
diff --git a/app/views/maps/_map.haml b/app/views/maps/_map.haml
index 1a26c23c..1aaa7586 100644
--- a/app/views/maps/_map.haml
+++ b/app/views/maps/_map.haml
@@ -10,10 +10,10 @@
     .map-auth{title: 'This map is private and not linked from the public listing.'}
       %i.bi.bi-lock-fill
 
-  - if map.user && avatar
-    - if map.user.image
-      = link_to = link_to url_for(params: { search: "user:#{map.user.id}"}), 'aria-label': "Browse maps by user #{map.user.email}" do
-        = image_tag avatar_url(map.user.image, 40), class: 'map-avatar', loading: 'lazy', alt: "", crossorigin: "anonymous"
+  - if map.owners.first && avatar
+    - if map.owners.first.image
+      = link_to = link_to url_for(params: { search: "user:#{map.owners.first.id}"}), 'aria-label': "Browse maps by user #{map.owners.first.email}" do
+        = image_tag avatar_url(map.owners.first.image, 40), class: 'map-avatar', loading: 'lazy', alt: "", crossorigin: "anonymous"
 
   - if clone
     = link_to copy_map_path(id: map.public_id), data: { turbo_method: :post, turbo_confirm: "Do you want to copy this map?" }, title: 'Copy map', class: 'map-copy' do
diff --git a/app/views/maps/modals/_share.haml b/app/views/maps/modals/_share.haml
index 36b85c8a..b1cf83d1 100644
--- a/app/views/maps/modals/_share.haml
+++ b/app/views/maps/modals/_share.haml
@@ -8,7 +8,7 @@
 
       .feature-section-card.pt-3
         - if @map_mode == 'rw'
-          - if @map.user
+          - if @map.owners.any?
             %p
               You can access this map under '#{link_to "Your maps", my_path}'.
           - else
@@ -27,7 +27,7 @@
               .form-floating.edit-ui
                 %label{ for: "map-edit-permissions"} Edit permissions:
                 %select.form-select#map-edit-permissions{ "name": "map-visibility", "aria-label": "Edit permissions", data: { action: "change->map--share#updateEditPermissions" }}
-                  - if @map.user
+                  - if @map.owners.any?
                     %option{ value: 'private' } Only you
                   %option{ value: 'link' } Everybody with the edit link
 
@@ -41,7 +41,7 @@
               .form-floating.edit-ui
                 %label{ for: "map-view-permissions"} View permissions:
                 %select.form-select#map-view-permissions{ "name": "map-visibility", "aria-label": "View permissions", data: { action: "change->map--share#updateViewPermissions" }}
-                  - if @map.user
+                  - if @map.owners.any?
                     %option{ value: 'private' } Only you
                   %option{ value: 'link' } Everybody with the view link
                   %option{ value: 'listed' } Everybody, map is shown on mapforge
diff --git a/app/views/shared/_header.html.haml b/app/views/shared/_header.html.haml
index 165dcb46..ac8213e3 100644
--- a/app/views/shared/_header.html.haml
+++ b/app/views/shared/_header.html.haml
@@ -41,7 +41,7 @@
             %li.nav-item
               = button_to my_path, { method: :get, class: 'mb-2 btn btn-green w-100 no-wrap' } do
                 %i.bi.bi-map.me-1
-                = "Your maps (#{@user.maps_count})"
+                = "Your maps (#{@user.owned_maps.count})"
 
           - if @user&.admin?
             %li.nav-item
@@ -88,7 +88,7 @@
           %li
             = link_to my_path, { class: 'dropdown-item' } do
               %i.bi.bi-map
-              = "Your maps (#{@user.maps_count})"
+              = "Your maps (#{@user.owned_maps.count})"
           %li
             = link_to logout_path, { class: 'dropdown-item', data: { turbo: false } } do
               %i.bi.bi-box-arrow-in-left
diff --git a/db/migrate/20260328003620_convert_to_multi_owner.rb b/db/migrate/20260328003620_convert_to_multi_owner.rb
new file mode 100644
index 00000000..27359c0f
--- /dev/null
+++ b/db/migrate/20260328003620_convert_to_multi_owner.rb
@@ -0,0 +1,29 @@
+class ConvertToMultiOwner < Mongoid::Migration
+  def self.up
+    # Maps with users: convert user_id to owner_ids array
+    Map.collection.update_many(
+      { user_id: { '$ne' => nil } },
+      [ { '$set' => { owner_ids: [ '$user_id' ] } } ]
+    )
+
+    # Anonymous maps: set empty owner_ids array
+    Map.collection.update_many(
+      { user_id: nil },
+      { '$set' => { owner_ids: [] } }
+    )
+
+    # Remove user_id field
+    # Map.collection.update_many({}, { '$unset' => { user_id: '' } })
+  end
+
+  def self.down
+    # Restore user_id from first owner
+    Map.collection.update_many(
+      { owner_ids: { '$ne' => [] } },
+      [ { '$set' => { user_id: { '$first' => '$owner_ids' } } } ]
+    )
+
+    # Remove owner_ids field
+    # Map.collection.update_many({}, { '$unset' => { owner_ids: '' } })
+  end
+end
diff --git a/engines/ulogger/app/controllers/api/ulogger_controller.rb b/engines/ulogger/app/controllers/api/ulogger_controller.rb
index 81817266..17176c2e 100644
--- a/engines/ulogger/app/controllers/api/ulogger_controller.rb
+++ b/engines/ulogger/app/controllers/api/ulogger_controller.rb
@@ -29,10 +29,11 @@ def addtrack
         @map = Map.find_by(private_id: $1)
       else
         session["track_name"] = params[:track]
-        @map = Map.create!(private_id: random_map_id, name: params[:track],
+        @map = Map.new(private_id: random_map_id, name: params[:track],
           view_permission: "link",
-          edit_permission: "link",
-          user: @user)
+          edit_permission: "link")
+        @map.add_owner(@user) if @user
+        @map.save!
       end
       if @map
         render json: { error: false, trackid:  @map.private_id.to_i }
@@ -118,7 +119,7 @@ def save_image(uploaded)
       filename = "ulogger-#{SecureRandom.hex(4)}.#{ext}"
 
       uid = Dragonfly.app.store(uploaded.tempfile, "name" => filename) # name needs to be a string here
-      Image.create(img_uid: uid, public_id: filename, user: @map.user)
+      Image.create(img_uid: uid, public_id: filename, user: @map.owners.first)
     end
 
     def image_properties(img)
diff --git a/spec/factories/maps.rb b/spec/factories/maps.rb
index a64bd9bd..5c74b995 100644
--- a/spec/factories/maps.rb
+++ b/spec/factories/maps.rb
@@ -7,10 +7,16 @@
 
     transient do
       features { nil }
+      owner { nil }
     end
 
     after :create do |map, evaluator|
       map.layers.first.update(features: evaluator.features)
+
+      # Add owner if provided
+      if evaluator.owner
+        map.add_owner(evaluator.owner)
+      end
     end
   end
 end
diff --git a/spec/features/map_permissions_spec.rb b/spec/features/map_permissions_spec.rb
index e540c61d..b57c3b9c 100644
--- a/spec/features/map_permissions_spec.rb
+++ b/spec/features/map_permissions_spec.rb
@@ -9,7 +9,7 @@
     end
 
     context "private" do
-      subject(:map) { create(:map, edit_permission: "private", user: user) }
+      subject(:map) { create(:map, edit_permission: "private", owners: [ user ]) }
 
       it "is not accessible via link" do
         expect(page).to have_current_path(maps_path)
@@ -37,7 +37,7 @@
     end
 
     context "private" do
-      subject(:map) { create(:map, view_permission: "private", user: user) }
+      subject(:map) { create(:map, view_permission: "private", owners: [ user ]) }
 
       it "is not accessible via link" do
         expect(page).to have_current_path(maps_path)
diff --git a/spec/features/map_share_spec.rb b/spec/features/map_share_spec.rb
index 582b7122..80b62ac7 100644
--- a/spec/features/map_share_spec.rb
+++ b/spec/features/map_share_spec.rb
@@ -1,7 +1,7 @@
 require "rails_helper"
 
 describe "Map" do
-  subject(:map) { create(:map, name: "Test Map", user: create(:user)) }
+  subject(:map) { create(:map, name: "Test Map", owners: [ create(:user) ]) }
 
   context "share links" do
     before do
@@ -33,7 +33,7 @@
   end
 
   context "export" do
-    subject(:map) { create(:map, user: create(:user), features: features) }
+    subject(:map) { create(:map, owners: [ create(:user) ], features: features) }
 
     let(:features) { create_list(:feature, 2, :line_string) }
 
@@ -49,7 +49,7 @@
   end
 
   context "export gpx" do
-    subject(:map) { create(:map, user: create(:user), features: features) }
+    subject(:map) { create(:map, owners: [ create(:user) ], features: features) }
 
     let(:features) {
       [ create(:feature, :line_string, coordinates: [ [ 11.041, 49.481 ], [ 11.056, 49.463 ] ]),
diff --git a/spec/features/map_view_spec.rb b/spec/features/map_view_spec.rb
index 0ae7bf12..d0a24ef2 100644
--- a/spec/features/map_view_spec.rb
+++ b/spec/features/map_view_spec.rb
@@ -173,7 +173,7 @@
   end
 
   context "as map owner / admin" do
-    let(:map) { create(:map, user: user) }
+    let(:map) { create(:map, owners: [ user ]) }
     let(:user) { create(:user) }
 
     before do
diff --git a/spec/features/maps_list_spec.rb b/spec/features/maps_list_spec.rb
index f1272d7b..49163bd9 100644
--- a/spec/features/maps_list_spec.rb
+++ b/spec/features/maps_list_spec.rb
@@ -25,7 +25,7 @@
   context "filter list" do
     let(:maps) {
       [ create(:map, name: "Map1", view_permission: "listed"),
-        create(:map, name: "Map2", user: user, view_permission: "listed") ]
+        create(:map, name: "Map2", owners: [ user ], view_permission: "listed") ]
     }
 
     it "searches in map names" do
diff --git a/spec/features/maps_my_list_spec.rb b/spec/features/maps_my_list_spec.rb
index 27e18b25..02c0b8fd 100644
--- a/spec/features/maps_my_list_spec.rb
+++ b/spec/features/maps_my_list_spec.rb
@@ -5,28 +5,28 @@
 
   before do
     allow_any_instance_of(ApplicationController).to receive(:session).and_return({ user_id: user.id })
-    create(:map, user: user)
+    create(:map, owners: [ user ])
     visit my_path
   end
 
   it "shows private links to maps" do
-    expect(page).to have_selector(:xpath, "//a[@href='/m/#{user.maps.first.private_id}']")
+    expect(page).to have_selector(:xpath, "//a[@href='/m/#{user.owned_maps.first.private_id}']")
   end
 
   it "can delete own map" do
-    expect(user.maps_count).to eq 1
+    expect(user.owned_maps.count).to eq 1
     expect(page).to have_css(".map-preview")
     accept_alert do
       find(".map-delete", match: :first).click
     end
     expect(page).not_to have_css(".map-preview")
-    expect(user.reload.maps_count).to eq 0
+    expect(user.reload.owned_maps.count).to eq 0
   end
 
   context "filter list" do
     before {
-      [ create(:map, user: user, name: "Map1"),
-        create(:map, user: user, name: "Map2") ]
+      [ create(:map, owners: [ user ], name: "Map1"),
+        create(:map, owners: [ user ], name: "Map2") ]
     }
 
     it "searches in map names" do
diff --git a/spec/models/map_spec.rb b/spec/models/map_spec.rb
index ec247511..a3667187 100644
--- a/spec/models/map_spec.rb
+++ b/spec/models/map_spec.rb
@@ -121,4 +121,52 @@
       expect(clone.features.map(&:id)).not_to match_array(subject.features.map(&:id))
     end
   end
+
+  describe 'multi-owner support' do
+    let(:user1) { create(:user) }
+    let(:user2) { create(:user) }
+    let(:map) { create(:map, owners: [ user1 ]) }
+
+    describe '#owned_by?' do
+      it 'returns true for owners' do
+        expect(map.owned_by?(user1)).to be true
+      end
+
+      it 'returns false for non-owners' do
+        expect(map.owned_by?(user2)).to be false
+      end
+    end
+
+    describe '#add_owner' do
+      it 'adds a new owner' do
+        expect { map.add_owner(user2) }.to change { map.owners.count }.by(1)
+      end
+
+      it 'is idempotent' do
+        map.add_owner(user2)
+        expect { map.add_owner(user2) }.not_to change { map.owners.count }
+      end
+    end
+
+    describe '#remove_owner' do
+      it 'removes an owner' do
+        map.add_owner(user2)
+        expect { map.remove_owner(user2) }.to change { map.owners.count }.by(-1)
+      end
+
+      it 'allows removing last owner (for anonymous maps)' do
+        expect { map.remove_owner(user1) }.to change { map.owners.count }.by(-1)
+        expect(map.owners).to be_empty
+      end
+    end
+
+    describe 'anonymous maps' do
+      it 'allows maps without owners' do
+        map = create(:map)
+        map.owners.clear
+        expect(map.owners).to be_empty
+        expect(map).to be_valid
+      end
+    end
+  end
 end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 2a8a2e90..1e9a03ed 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -4,7 +4,7 @@
   subject(:user) { create :user }
 
   it "with defaults" do
-    expect(user.maps_count).to be_zero
+    expect(user.owned_maps.count).to be_zero
     expect(user.images_count).to be_zero
     expect(user.recent_map_ids).to eq([])
   end