diff --git a/src/utils/api-fetch.js b/src/utils/api-fetch.js
index 590a44780..d5fee136d 100644
--- a/src/utils/api-fetch.js
+++ b/src/utils/api-fetch.js
@@ -109,10 +109,11 @@ function apiPathModifierMiddleware( options, next ) {
 		).test( options.path ) || /\/sites\/[^/]+\//.test( options.path );
 
 	if ( isEligiblePath && ! alreadyHasSiteNamespace ) {
-		// Insert the API namespace after the first two path segments.
+		// Insert the API namespace after the first two path segments, with a
+		// single trailing slash.
 		options.path = options.path.replace(
 			/^(?<apiPath>\/?(?:[\w.-]+\/){2})/,
-			`$<apiPath>${ siteApiNamespace[ 0 ] }`
+			`$<apiPath>${ siteApiNamespace[ 0 ].replace( /\/+$/, '' ) }/`
 		);
 	}
 
diff --git a/src/utils/api-fetch.test.js b/src/utils/api-fetch.test.js
index 63cac028b..f4025333a 100644
--- a/src/utils/api-fetch.test.js
+++ b/src/utils/api-fetch.test.js
@@ -212,6 +212,33 @@ describe( 'api-fetch credentials handling', () => {
 			expect( requestedUrl() ).toContain( '/wp/v2/sites/123/posts' );
 		} );
 
+		it( 'inserts a namespace configured without a trailing slash', async () => {
+			// Both forms are supported; the native URL builders normalize them
+			// identically. Without normalizing here the namespace would run into
+			// the following segment: `/wp/v2/sites/123posts`.
+			bridge.getGBKit.mockReturnValue( {
+				siteApiRoot: 'https://example.com/wp-json/',
+				siteApiNamespace: [ 'sites/123' ],
+				namespaceExcludedPaths: [],
+			} );
+
+			await apiFetch( { path: '/wp/v2/posts' } ).catch( () => {} );
+
+			expect( requestedUrl() ).toContain( '/wp/v2/sites/123/posts' );
+		} );
+
+		it( 'does not double the slash on a namespace that already ends with one', async () => {
+			bridge.getGBKit.mockReturnValue( {
+				siteApiRoot: 'https://example.com/wp-json/',
+				siteApiNamespace: [ 'sites/123/' ],
+				namespaceExcludedPaths: [],
+			} );
+
+			await apiFetch( { path: '/wp/v2/posts' } ).catch( () => {} );
+
+			expect( requestedUrl() ).not.toContain( 'sites/123//' );
+		} );
+
 		it( 'leaves the path alone when it already carries the namespace', async () => {
 			bridge.getGBKit.mockReturnValue( {
 				siteApiRoot: 'https://example.com/wp-json/',