diff --git a/CHANGELOG.md b/CHANGELOG.md
index 62e8d8666..12bed920d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,7 +10,7 @@ _None_
 
 ### New Features
 
-_None_
+- New `macos_verify_code_signing` action, asserting that macOS artifacts are signed, signed by the expected authority, accepted by Gatekeeper, and have a notarization ticket stapled to them. Handles `.app` bundles and `.dmg` disk images, picking the checks that apply to each. [#757]
 
 ### Bug Fixes
 
diff --git a/lib/fastlane/plugin/wpmreleasetoolkit/actions/macos/macos_verify_code_signing.rb b/lib/fastlane/plugin/wpmreleasetoolkit/actions/macos/macos_verify_code_signing.rb
new file mode 100644
index 000000000..1824b0bcc
--- /dev/null
+++ b/lib/fastlane/plugin/wpmreleasetoolkit/actions/macos/macos_verify_code_signing.rb
@@ -0,0 +1,143 @@
+# frozen_string_literal: true
+
+require 'fastlane/action'
+require 'fastlane_core/ui/ui'
+
+module Fastlane
+  module Actions
+    class MacosVerifyCodeSigningAction < Action
+      def self.run(params)
+        paths = Array(params[:artifact_path])
+        UI.user_error!('No artifact to verify: `artifact_path` is empty') if paths.empty?
+
+        paths.each do |path|
+          UI.user_error!("There is no artifact at #{path}") unless File.exist?(path)
+
+          UI.message("Verifying #{path}")
+
+          case File.extname(path).downcase
+          when '.app'
+            verify_app_bundle(path: path, expected_authority: params[:expected_authority], verify_notarization: params[:verify_notarization])
+          when '.dmg'
+            verify_disk_image(path: path, expected_authority: params[:expected_authority], verify_notarization: params[:verify_notarization])
+          else
+            UI.user_error!("Don't know how to verify #{path}. Supported artifacts are `.app` bundles and `.dmg` disk images")
+          end
+        end
+
+        UI.success("Verified #{paths.length} artifact(s)")
+      end
+
+      def self.verify_app_bundle(path:, expected_authority:, verify_notarization:)
+        verify!("The code signature of #{path} is not valid", 'codesign', '--verify', '--deep', '--strict', '--verbose=2', path)
+        verify_authority!(path: path, expected_authority: expected_authority) unless expected_authority.nil?
+
+        return unless verify_notarization
+
+        verify!("#{path} was rejected by Gatekeeper", 'spctl', '--assess', '--type', 'execute', '--verbose=2', path)
+        verify!("#{path} has no notarization ticket stapled to it", 'xcrun', 'stapler', 'validate', path)
+      end
+
+      # Unlike an app bundle, a disk image is often not signed at all — `electron-builder`, for one,
+      # only signs the app inside it. Gatekeeper then rejects the image itself with `no usable
+      # signature` even when it carries a notarization ticket, so the stapled ticket is the only
+      # check that means anything for an unsigned image.
+      #
+      def self.verify_disk_image(path:, expected_authority:, verify_notarization:)
+        # Said up front because `sh` logs a non-zero exit in red regardless of it being handled,
+        # which reads as a broken build in the CI log of an otherwise passing job.
+        UI.message("Checking whether #{path} is signed. Disk images usually aren't, so a `codesign` failure due to the image being unsigned is expected and tolerated (other signature failures will still fail).")
+
+        if signed?(path)
+          verify_authority!(path: path, expected_authority: expected_authority) unless expected_authority.nil?
+          verify!("#{path} was rejected by Gatekeeper", 'spctl', '--assess', '--type', 'open', '--context', 'context:primary-signature', '--verbose=2', path) if verify_notarization
+        else
+          UI.important("#{path} is not signed — skipping its signature checks. Only the app it contains carries a signature.")
+        end
+
+        verify!("#{path} has no notarization ticket stapled to it", 'xcrun', 'stapler', 'validate', path) if verify_notarization
+      end
+
+      # Distinguishes an artifact that carries no signature at all from one whose signature is
+      # broken: the former is expected for a disk image, the latter always a failure.
+      #
+      def self.signed?(path)
+        exitstatus, output = sh('codesign', '--verify', '--strict', '--verbose=2', path) { |status, result, _| [status.exitstatus, result] }
+
+        return true if exitstatus.zero?
+        return false if output.include?('not signed at all')
+
+        UI.user_error!("The code signature of #{path} is not valid:\n#{output}")
+      end
+
+      def self.verify!(error_message, *command)
+        sh(*command, error_callback: ->(_) { UI.user_error!(error_message) })
+      end
+
+      def self.verify_authority!(path:, expected_authority:)
+        details = sh('codesign', '--display', '--verbose=2', path)
+        return if details.include?("Authority=#{expected_authority}")
+
+        UI.user_error!("#{path} is not signed by '#{expected_authority}':\n#{details}")
+      end
+
+      #####################################################
+      # @!group Documentation
+      #####################################################
+
+      def self.description
+        'Verify that macOS artifacts are properly code signed and notarized'
+      end
+
+      def self.details
+        <<~DETAILS
+          Verify that the given macOS artifacts are signed, and optionally notarized.
+
+          The checks that apply are picked from the artifact's extension:
+
+          - `.app` — the signature is valid and satisfies its designated requirement, Gatekeeper accepts
+            the bundle for execution, and a notarization ticket is stapled to it.
+          - `.dmg` — if the image is signed, the signature is valid (and can be checked against the expected authority) and Gatekeeper accepts opening it; when `verify_notarization` is true, a notarization ticket is stapled to the image.
+        DETAILS
+      end
+
+      def self.available_options
+        [
+          FastlaneCore::ConfigItem.new(
+            key: :artifact_path,
+            description: 'The path, or list of paths, to the `.app` bundle(s) or `.dmg` disk image(s) to verify',
+            is_string: false,
+            verify_block: proc do |value|
+              next if value.is_a?(String)
+              next if value.is_a?(Array) && value.all?(String)
+
+              UI.user_error!('`artifact_path` must be a String or an Array of Strings')
+            end
+          ),
+          FastlaneCore::ConfigItem.new(
+            key: :expected_authority,
+            description: 'The signing authority the artifact is expected to be signed by, e.g. `Developer ID Application: ACME, Inc. (ABCDE12345)`. ' \
+                         + 'When omitted, any valid signature is accepted',
+            type: String,
+            optional: true,
+            default_value: nil
+          ),
+          FastlaneCore::ConfigItem.new(
+            key: :verify_notarization,
+            description: 'Whether to also assert that the artifact is accepted by Gatekeeper and has a notarization ticket stapled to it',
+            type: Boolean,
+            default_value: true
+          ),
+        ]
+      end
+
+      def self.authors
+        ['Automattic']
+      end
+
+      def self.is_supported?(platform)
+        platform == :mac
+      end
+    end
+  end
+end
diff --git a/spec/macos_verify_code_signing_spec.rb b/spec/macos_verify_code_signing_spec.rb
new file mode 100644
index 000000000..dca333e4f
--- /dev/null
+++ b/spec/macos_verify_code_signing_spec.rb
@@ -0,0 +1,229 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Fastlane::Actions::MacosVerifyCodeSigningAction do
+  let(:authority) { 'Developer ID Application: Automattic, Inc. (ABCDE12345)' }
+
+  # Runs the action against artifacts that exist on disk, so that the existence
+  # check doesn't get in the way of asserting on the commands the action runs.
+  #
+  def with_artifacts(*names)
+    in_tmp_dir do |tmp_dir|
+      paths = names.map do |name|
+        path = File.join(tmp_dir, name)
+        # A `.app` is a directory and a `.dmg` a file, but the action only calls
+        # `File.exist?`, so either satisfies it.
+        FileUtils.mkdir_p(path)
+        path
+      end
+      yield(paths)
+    end
+  end
+
+  def expect_codesign_verify_deep(path, exitstatus: 0)
+    expect_shell_command('codesign', '--verify', '--deep', '--strict', '--verbose=2', path, exitstatus: exitstatus)
+  end
+
+  def expect_codesign_verify(path, exitstatus: 0, output: '')
+    expect_shell_command('codesign', '--verify', '--strict', '--verbose=2', path, exitstatus: exitstatus, output: output)
+  end
+
+  def expect_codesign_display(path, authority:)
+    expect_shell_command('codesign', '--display', '--verbose=2', path, output: "Executable=#{path}\nAuthority=#{authority}\n")
+  end
+
+  def expect_gatekeeper_assess_execute(path, exitstatus: 0)
+    expect_shell_command('spctl', '--assess', '--type', 'execute', '--verbose=2', path, exitstatus: exitstatus)
+  end
+
+  def expect_gatekeeper_assess_open(path, exitstatus: 0)
+    expect_shell_command('spctl', '--assess', '--type', 'open', '--context', 'context:primary-signature', '--verbose=2', path, exitstatus: exitstatus)
+  end
+
+  def expect_stapler_validate(path, exitstatus: 0)
+    expect_shell_command('xcrun', 'stapler', 'validate', path, exitstatus: exitstatus)
+  end
+
+  before do
+    allow_fastlane_action_sh
+  end
+
+  describe 'app bundles' do
+    it 'verifies the signature, Gatekeeper acceptance and stapled ticket' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path)
+        expect_gatekeeper_assess_execute(path)
+        expect_stapler_validate(path)
+
+        run_described_fastlane_action(artifact_path: path)
+      end
+    end
+
+    it 'skips the notarization checks when `verify_notarization` is `false`' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path)
+        expect(Open3).not_to receive(:popen2e).with('spctl', any_args)
+        expect(Open3).not_to receive(:popen2e).with('xcrun', any_args)
+
+        run_described_fastlane_action(artifact_path: path, verify_notarization: false)
+      end
+    end
+
+    it 'passes when signed by the expected authority' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path)
+        expect_codesign_display(path, authority: authority)
+        expect_gatekeeper_assess_execute(path)
+        expect_stapler_validate(path)
+
+        run_described_fastlane_action(artifact_path: path, expected_authority: authority)
+      end
+    end
+
+    it 'fails when signed by a different authority' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path)
+        expect_codesign_display(path, authority: 'Apple Development: Someone Else (ZZZZZ99999)')
+
+        expect { run_described_fastlane_action(artifact_path: path, expected_authority: authority) }
+          .to raise_error(FastlaneCore::Interface::FastlaneError, /is not signed by '#{Regexp.escape(authority)}'/)
+      end
+    end
+
+    it 'fails when the signature is not valid' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path, exitstatus: 1)
+
+        expect { run_described_fastlane_action(artifact_path: path) }
+          .to raise_error(FastlaneCore::Interface::FastlaneError, /The code signature of .*Test\.app is not valid/)
+      end
+    end
+
+    it 'fails when Gatekeeper rejects it' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path)
+        expect_gatekeeper_assess_execute(path, exitstatus: 3)
+
+        expect { run_described_fastlane_action(artifact_path: path) }
+          .to raise_error(FastlaneCore::Interface::FastlaneError, /Test\.app was rejected by Gatekeeper/)
+      end
+    end
+
+    it 'fails when it has no notarization ticket stapled' do
+      with_artifacts('Test.app') do |(path)|
+        expect_codesign_verify_deep(path)
+        expect_gatekeeper_assess_execute(path)
+        expect_stapler_validate(path, exitstatus: 65)
+
+        expect { run_described_fastlane_action(artifact_path: path) }
+          .to raise_error(FastlaneCore::Interface::FastlaneError, /Test\.app has no notarization ticket stapled to it/)
+      end
+    end
+  end
+
+  describe 'disk images' do
+    # `electron-builder` signs the app inside the image but not the image itself,
+    # which is what our own `.dmg` artifacts look like.
+    it 'checks only the stapled ticket when the image is not signed' do
+      with_artifacts('Test.dmg') do |(path)|
+        expect_codesign_verify(path, exitstatus: 1, output: "#{path}: code object is not signed at all\n")
+        expect_stapler_validate(path)
+        expect(Open3).not_to receive(:popen2e).with('spctl', any_args)
+        expect(FastlaneCore::UI).to receive(:important).with(/is not signed — skipping its signature checks/)
+
+        run_described_fastlane_action(artifact_path: path)
+      end
+    end
+
+    it 'does not assert the authority of an unsigned image' do
+      with_artifacts('Test.dmg') do |(path)|
+        expect_codesign_verify(path, exitstatus: 1, output: "#{path}: code object is not signed at all\n")
+        expect_stapler_validate(path)
+        expect(Open3).not_to receive(:popen2e).with('codesign', '--display', any_args)
+
+        run_described_fastlane_action(artifact_path: path, expected_authority: authority)
+      end
+    end
+
+    it 'checks Gatekeeper and the authority when the image is signed' do
+      with_artifacts('Test.dmg') do |(path)|
+        expect_codesign_verify(path)
+        expect_codesign_display(path, authority: authority)
+        expect_gatekeeper_assess_open(path)
+        expect_stapler_validate(path)
+
+        run_described_fastlane_action(artifact_path: path, expected_authority: authority)
+      end
+    end
+
+    it 'fails when the image carries a broken signature' do
+      with_artifacts('Test.dmg') do |(path)|
+        expect_codesign_verify(path, exitstatus: 1, output: "#{path}: invalid signature (code or signature have been modified)\n")
+
+        expect { run_described_fastlane_action(artifact_path: path) }
+          .to raise_error(FastlaneCore::Interface::FastlaneError, /The code signature of .*Test\.dmg is not valid/)
+      end
+    end
+
+    it 'fails when it has no notarization ticket stapled' do
+      with_artifacts('Test.dmg') do |(path)|
+        expect_codesign_verify(path, exitstatus: 1, output: "#{path}: code object is not signed at all\n")
+        expect_stapler_validate(path, exitstatus: 65)
+
+        expect { run_described_fastlane_action(artifact_path: path) }
+          .to raise_error(FastlaneCore::Interface::FastlaneError, /Test\.dmg has no notarization ticket stapled to it/)
+      end
+    end
+
+    it 'skips every check but the signature when `verify_notarization` is `false`' do
+      with_artifacts('Test.dmg') do |(path)|
+        expect_codesign_verify(path)
+        expect(Open3).not_to receive(:popen2e).with('spctl', any_args)
+        expect(Open3).not_to receive(:popen2e).with('xcrun', any_args)
+
+        run_described_fastlane_action(artifact_path: path, verify_notarization: false)
+      end
+    end
+  end
+
+  it 'verifies every artifact when given a list of paths' do
+    with_artifacts('One.app', 'Two.dmg') do |(app_path, dmg_path)|
+      expect_codesign_verify_deep(app_path)
+      expect_gatekeeper_assess_execute(app_path)
+      expect_stapler_validate(app_path)
+
+      expect_codesign_verify(dmg_path, exitstatus: 1, output: "#{dmg_path}: code object is not signed at all\n")
+      expect_stapler_validate(dmg_path)
+
+      run_described_fastlane_action(artifact_path: [app_path, dmg_path])
+    end
+  end
+
+  it 'fails on an artifact it does not know how to verify' do
+    with_artifacts('Test.zip') do |(path)|
+      expect { run_described_fastlane_action(artifact_path: path) }
+        .to raise_error(FastlaneCore::Interface::FastlaneError, /Don't know how to verify .*Test\.zip/)
+    end
+  end
+
+  it 'fails when there is no artifact at the given path' do
+    expect { run_described_fastlane_action(artifact_path: '/path/to/Missing.app') }
+      .to raise_error(FastlaneCore::Interface::FastlaneError, %r{There is no artifact at /path/to/Missing\.app})
+  end
+
+  it 'fails when given an empty list of paths' do
+    expect { run_described_fastlane_action(artifact_path: []) }
+      .to raise_error(FastlaneCore::Interface::FastlaneError, /`artifact_path` is empty/)
+  end
+
+  it 'fails when `artifact_path` is neither a String nor an Array' do
+    expect { run_described_fastlane_action(artifact_path: 42) }
+      .to raise_error(FastlaneCore::Interface::FastlaneError, /`artifact_path` must be a String or an Array of Strings/)
+  end
+
+  it 'fails when `artifact_path` is an Array of something other than Strings' do
+    expect { run_described_fastlane_action(artifact_path: [42]) }
+      .to raise_error(FastlaneCore::Interface::FastlaneError, /`artifact_path` must be a String or an Array of Strings/)
+  end
+end