diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index 6e06f1563c50c..78225e87e23da 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -459,6 +459,10 @@ public function create_item( $request ) {
 			// Disable server-side EXIF rotation so the client can handle it.
 			// This preserves the original orientation value in the metadata.
 			add_filter( 'wp_image_maybe_exif_rotate', '__return_false', 100 );
+			// Disable server-side "big image" downscaling; the client supplies its
+			// own scaled version via the sideload endpoint. Scaling here would
+			// create a conflicting "-scaled" file and orphan the full-size upload.
+			add_filter( 'big_image_size_threshold', '__return_false', 100 );
 		}
 
 		// Handle convert_format parameter.
@@ -691,6 +695,7 @@ private function remove_client_side_media_processing_filters(): void {
 		remove_filter( 'fallback_intermediate_image_sizes', '__return_empty_array', 100 );
 		remove_filter( 'wp_image_maybe_exif_rotate', '__return_false', 100 );
 		remove_filter( 'image_editor_output_format', '__return_empty_array', 100 );
+		remove_filter( 'big_image_size_threshold', '__return_false', 100 );
 	}
 
 	/**
diff --git a/tests/phpunit/tests/rest-api/rest-attachments-controller.php b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
index 90899df850d47..b79ee71e5ee71 100644
--- a/tests/phpunit/tests/rest-api/rest-attachments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
@@ -3875,6 +3875,71 @@ public function test_sideload_scaled_image() {
 		$this->assertGreaterThan( 0, $metadata['filesize'], 'Filesize should be positive.' );
 	}
 
+	/**
+	 * When the client generates sub-sizes (generate_sub_sizes is false), the
+	 * server must not perform its own "big image" downscaling on upload.
+	 *
+	 * Otherwise the server creates a `-scaled` file and records the upload as
+	 * `original_image`. The client's subsequent scaled sideload then collides
+	 * with that `-scaled` file and is renamed `-scaled-1`, the thumbnails
+	 * inherit the numbered name, and the server-generated full-size file is
+	 * left orphaned on disk.
+	 *
+	 * @ticket 65708
+	 * @requires function imagejpeg
+	 */
+	public function test_create_item_skips_big_image_scaling_when_client_generates_sub_sizes() {
+		$this->enable_client_side_media_processing();
+
+		wp_set_current_user( self::$author_id );
+
+		// Force the threshold below the image's dimensions so scaling would be
+		// triggered were it not suppressed for client-side processing.
+		add_filter(
+			'big_image_size_threshold',
+			static function () {
+				return 1000;
+			}
+		);
+
+		// Upload a large image with the client handling sub-size generation.
+		$request = new WP_REST_Request( 'POST', '/wp/v2/media' );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=33772.jpg' );
+		$request->set_param( 'generate_sub_sizes', false );
+		$request->set_body( file_get_contents( DIR_TESTDATA . '/images/33772.jpg' ) );
+		$response      = rest_get_server()->dispatch( $request );
+		$data          = $response->get_data();
+		$attachment_id = $data['id'];
+
+		$this->assertSame( 201, $response->get_status(), 'Uploading the image should succeed.' );
+
+		// The uploaded full-size image should be stored untouched: no
+		// server-side "-scaled" file and no original_image swap.
+		$original_file      = get_attached_file( $attachment_id, true );
+		$original_basename  = wp_basename( $original_file );
+		$original_name_stem = pathinfo( $original_basename, PATHINFO_FILENAME );
+		$this->assertStringNotContainsString( '-scaled', $original_basename, 'The server should not create a -scaled file when the client generates sub-sizes.' );
+
+		$metadata = wp_get_attachment_metadata( $attachment_id );
+		$this->assertArrayNotHasKey( 'original_image', $metadata, 'The server should not record an original_image when it does not scale the upload.' );
+
+		// The client's scaled sideload should now record the untouched upload as
+		// original_image and keep the -scaled name without a numeric suffix.
+		$request = new WP_REST_Request( 'POST', "/wp/v2/media/{$attachment_id}/sideload" );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', "attachment; filename={$original_name_stem}-scaled.jpg" );
+		$request->set_param( 'image_size', 'scaled' );
+		$request->set_body( file_get_contents( DIR_TESTDATA . '/images/33772.jpg' ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertSame( 200, $response->get_status(), 'Sideloading the scaled image should succeed.' );
+
+		$sub_size = $response->get_data();
+		$this->assertSame( $original_basename, $sub_size['original_image'], 'The untouched upload should be recorded as original_image.' );
+		$this->assertSame( "{$original_name_stem}-scaled.jpg", wp_basename( $sub_size['file'] ), 'The scaled sideload should keep the -scaled name without a numeric collision suffix.' );
+	}
+
 	/**
 	 * Tests that sideloading scaled image requires authentication.
 	 *