diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index d999c7bfeb..b434ae52e4 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -4142,6 +4142,8 @@ function wp_insert_post( $postarr, $wp_error = false, $fire_after_hooks = true )
 	if ( ! empty( $postarr['post_category'] ) ) {
 		// Filter out empty terms.
 		$post_category = array_filter( $postarr['post_category'] );
+	} elseif ( $update && ! isset( $postarr['post_category'] ) ) {
+		$post_category = $post_before->post_category;
 	}
 
 	// Make sure we set a valid category.
diff --git a/tests/phpunit/tests/post/wpInsertPost.php b/tests/phpunit/tests/post/wpInsertPost.php
index 5a8ce8f2d9..a86d6560d1 100644
--- a/tests/phpunit/tests/post/wpInsertPost.php
+++ b/tests/phpunit/tests/post/wpInsertPost.php
@@ -876,6 +876,51 @@ class Tests_Post_wpInsertPost extends WP_UnitTestCase {
 		$this->assertSame( $changeset_data, json_decode( get_post( $post_id )->post_content, true ) );
 	}
 
+	/**
+	 * @ticket 19954
+	 */
+	function test_updating_a_post_should_not_trash_categories() {
+		// Create a category and attach it to a new post.
+		$term_id = self::factory()->term->create(
+			array(
+				'name'     => 'Term',
+				'taxonomy' => 'category',
+			)
+		);
+
+		$post_id = self::factory()->post->create(
+			array(
+				'post_type'     => 'post',
+				'post_title'    => 'Post with categories',
+				'post_status'   => 'publish',
+				'post_category' => array( $term_id ),
+			)
+		);
+
+		// Validate that the term got assigned.
+		$assigned_terms = wp_get_object_terms( array( $post_id ), array( 'category' ), array() );
+		$this->assertCount( 1, $assigned_terms );
+		$this->assertEquals( $term_id, $assigned_terms[0]->term_id );
+
+		// Update the post with no changes.
+		$post = get_post( $post_id );
+		wp_insert_post( $post );
+
+		// Validate the term is still assigned.
+		$assigned_terms = wp_get_object_terms( array( $post_id ), array( 'category' ), array() );
+		$this->assertCount( 1, $assigned_terms );
+		$this->assertEquals( $term_id, $assigned_terms[0]->term_id );
+
+		// Remove the term from the post.
+		$post->post_category = array();
+		wp_insert_post( $post );
+		$assigned_terms = wp_get_object_terms( array( $post_id ), array( 'category' ), array() );
+
+		// Validate that the post has had the default category assigned again.
+		$this->assertCount( 1, $assigned_terms );
+		$this->assertEquals( get_option( 'default_category' ), $assigned_terms[0]->term_id );
+	}
+
 	/**
 	 * @ticket 48113
 	 */