diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 4f46eb674..4d1383fbd 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -48,6 +48,8 @@ registry/[namespace]/
 
 For example: `/registry/your-username/modules/` and `/registry/your-username/templates/`. If a namespace is taken, choose a different unique namespace, but you can still use any display name on the Registry website.
 
+Namespace directory names must be **lowercase**, may only contain letters, numbers, and hyphens, and must start and end with a letter or number. The namespace becomes part of the case-sensitive module source path (`registry.coder.com/[namespace]/[module]/coder`), so lowercase keeps those paths predictable. A few namespaces created before this rule keep their mixed-case names because renaming them would break existing module references.
+
 ### Images and Icons
 
 - **Namespace avatars**: Must be named `avatar.png` or `avatar.svg` in `/registry/[namespace]/.images/`
diff --git a/cmd/readmevalidation/repostructure.go b/cmd/readmevalidation/repostructure.go
index 984218869..09782d376 100644
--- a/cmd/readmevalidation/repostructure.go
+++ b/cmd/readmevalidation/repostructure.go
@@ -16,6 +16,34 @@ var supportedUserNameSpaceDirectories = append(supportedResourceTypes, ".images"
 // validNameRe validates that names contain only alphanumeric characters and hyphens
 var validNameRe = regexp.MustCompile(`^[a-zA-Z0-9](?:[a-zA-Z0-9-]*[a-zA-Z0-9])?$`)
 
+// validNamespaceRe validates that a namespace directory name is lowercase, contains only alphanumeric characters and
+// hyphens, and starts and ends with an alphanumeric character. A namespace becomes part of the case-sensitive module
+// source path (registry.coder.com/<namespace>/<module>/coder), so mixed case produces paths that are inconsistent and
+// easy to mistype.
+var validNamespaceRe = regexp.MustCompile(`^[a-z0-9](?:[a-z0-9-]*[a-z0-9])?$`)
+
+// grandfatheredMixedCaseNamespaces lists the namespaces that existed before the lowercase rule and cannot be renamed
+// yet. Both have published modules whose source paths are case-sensitive, and the registry server has no alias
+// mechanism for the old path. They are exempt until that migration happens. Do not add new entries.
+var grandfatheredMixedCaseNamespaces = []string{
+	"AJ0070",
+	"BenraouaneSoufiane",
+}
+
+// validateNamespaceName validates the directory name of a single namespace under /registry.
+func validateNamespaceName(namespaceName string) error {
+	if slices.Contains(grandfatheredMixedCaseNamespaces, namespaceName) {
+		return nil
+	}
+	if validNamespaceRe.MatchString(namespaceName) {
+		return nil
+	}
+	// If the lowercased name is valid, case was the only problem, so point at the name to use.
+	if lowercased := strings.ToLower(namespaceName); validNamespaceRe.MatchString(lowercased) {
+		return xerrors.Errorf("namespace name must be lowercase (use %q)", lowercased)
+	}
+	return xerrors.New("namespace name must contain only lowercase alphanumeric characters and hyphens, starting and ending with an alphanumeric character")
+}
 
 // validateCoderResourceSubdirectory validates that the structure of a module or template within a namespace follows all
 // expected file conventions
@@ -91,8 +119,8 @@ func validateRegistryDirectory() []error {
 		}
 
 		// Validate namespace name
-		if !validNameRe.MatchString(nDir.Name()) {
-			allErrs = append(allErrs, xerrors.Errorf("%q: namespace name contains invalid characters (only alphanumeric characters and hyphens are allowed)", namespacePath))
+		if err := validateNamespaceName(nDir.Name()); err != nil {
+			allErrs = append(allErrs, xerrors.Errorf("%q: %w", namespacePath, err))
 			continue
 		}
 
diff --git a/cmd/readmevalidation/repostructure_test.go b/cmd/readmevalidation/repostructure_test.go
new file mode 100644
index 000000000..953ddd93f
--- /dev/null
+++ b/cmd/readmevalidation/repostructure_test.go
@@ -0,0 +1,55 @@
+package main
+
+import "testing"
+
+func TestValidateNamespaceName(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name       string
+		shouldPass bool
+	}{
+		// Lowercase namespaces are always allowed.
+		{name: "coder", shouldPass: true},
+		{name: "coder-labs", shouldPass: true},
+		{name: "aj0070", shouldPass: true},
+		{name: "user123", shouldPass: true},
+		{name: "excellencedev", shouldPass: true},
+		{name: "iamtaochen", shouldPass: true},
+
+		// Mixed-case namespaces that predate the rule stay allowed.
+		{name: "AJ0070", shouldPass: true},
+		{name: "BenraouaneSoufiane", shouldPass: true},
+
+		// New mixed-case namespaces are rejected.
+		{name: "Coder", shouldPass: false},
+		{name: "CoderLabs", shouldPass: false},
+		{name: "coder-Labs", shouldPass: false},
+		{name: "Excellencedev", shouldPass: false},
+		{name: "IamTaoChen", shouldPass: false},
+
+		// Other invalid names are still rejected.
+		{name: "", shouldPass: false},
+		{name: "-coder", shouldPass: false},
+		{name: "coder-", shouldPass: false},
+		{name: "coder_labs", shouldPass: false},
+		{name: "coder labs", shouldPass: false},
+		{name: "coder.labs", shouldPass: false},
+		{name: "Coder_Labs", shouldPass: false},
+		{name: "-Coder", shouldPass: false},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := validateNamespaceName(tc.name)
+			if tc.shouldPass && err != nil {
+				t.Errorf("expected %q to be a valid namespace name, got error: %v", tc.name, err)
+			}
+			if !tc.shouldPass && err == nil {
+				t.Errorf("expected %q to be an invalid namespace name, got no error", tc.name)
+			}
+		})
+	}
+}
diff --git a/registry/Excellencedev/.images/avatar.png b/registry/excellencedev/.images/avatar.png
similarity index 100%
rename from registry/Excellencedev/.images/avatar.png
rename to registry/excellencedev/.images/avatar.png
diff --git a/registry/Excellencedev/README.md b/registry/excellencedev/README.md
similarity index 100%
rename from registry/Excellencedev/README.md
rename to registry/excellencedev/README.md
diff --git a/registry/Excellencedev/templates/hetzner-linux/README.md b/registry/excellencedev/templates/hetzner-linux/README.md
similarity index 100%
rename from registry/Excellencedev/templates/hetzner-linux/README.md
rename to registry/excellencedev/templates/hetzner-linux/README.md
diff --git a/registry/Excellencedev/templates/hetzner-linux/cloud-config.yaml.tftpl b/registry/excellencedev/templates/hetzner-linux/cloud-config.yaml.tftpl
similarity index 100%
rename from registry/Excellencedev/templates/hetzner-linux/cloud-config.yaml.tftpl
rename to registry/excellencedev/templates/hetzner-linux/cloud-config.yaml.tftpl
diff --git a/registry/Excellencedev/templates/hetzner-linux/main.tf b/registry/excellencedev/templates/hetzner-linux/main.tf
similarity index 100%
rename from registry/Excellencedev/templates/hetzner-linux/main.tf
rename to registry/excellencedev/templates/hetzner-linux/main.tf
diff --git a/registry/IamTaoChen/.images/avatar.png b/registry/iamtaochen/.images/avatar.png
similarity index 100%
rename from registry/IamTaoChen/.images/avatar.png
rename to registry/iamtaochen/.images/avatar.png
diff --git a/registry/IamTaoChen/README.md b/registry/iamtaochen/README.md
similarity index 100%
rename from registry/IamTaoChen/README.md
rename to registry/iamtaochen/README.md
diff --git a/registry/IamTaoChen/templates/ssh-linux/README.md b/registry/iamtaochen/templates/ssh-linux/README.md
similarity index 100%
rename from registry/IamTaoChen/templates/ssh-linux/README.md
rename to registry/iamtaochen/templates/ssh-linux/README.md
diff --git a/registry/IamTaoChen/templates/ssh-linux/main.tf b/registry/iamtaochen/templates/ssh-linux/main.tf
similarity index 100%
rename from registry/IamTaoChen/templates/ssh-linux/main.tf
rename to registry/iamtaochen/templates/ssh-linux/main.tf