diff --git src/wp-includes/taxonomy.php src/wp-includes/taxonomy.php
index 7d49a54..eca8c61 100644
--- src/wp-includes/taxonomy.php
+++ src/wp-includes/taxonomy.php
@@ -3353,8 +3353,8 @@ function wp_update_term( $term_id, $taxonomy, $args = array() ) {
 	$parent = apply_filters( 'wp_update_term_parent', $args['parent'], $term_id, $taxonomy, $parsed_args, $args );
 
 	// Check for duplicate slug
-	$id = $wpdb->get_var( $wpdb->prepare( "SELECT term_id FROM $wpdb->terms WHERE slug = %s", $slug ) );
-	if ( $id && ($id != $term_id) ) {
+	$duplicate = get_term_by( 'slug', $slug, $taxonomy );
+	if ( $duplicate && $duplicate->term_id != $term_id ) {
 		// If an empty slug was passed or the parent changed, reset the slug to something unique.
 		// Otherwise, bail.
 		if ( $empty_slug || ( $parent != $term['parent']) )
diff --git tests/phpunit/tests/term.php tests/phpunit/tests/term.php
index 2cf5fb2..456dcd0 100644
--- tests/phpunit/tests/term.php
+++ tests/phpunit/tests/term.php
@@ -640,7 +640,7 @@ class Tests_Term extends WP_UnitTestCase {
 	/**
 	 * @ticket 5809
 	 */
-	public function test_wp_update_term_duplicate_slug_same_taxonomy() {
+	public function test_wp_update_term_should_not_create_duplicate_slugs_within_the_same_taxonomy() {
 		register_taxonomy( 'wptests_tax', 'post' );
 
 		$t1 = $this->factory->term->create( array(
@@ -650,7 +650,7 @@ class Tests_Term extends WP_UnitTestCase {
 		) );
 
 		$t2 = $this->factory->term->create( array(
-			'name' => 'Foo',
+			'name' => 'Bar',
 			'slug' => 'bar',
 			'taxonomy' => 'wptests_tax',
 		) );
@@ -666,7 +666,7 @@ class Tests_Term extends WP_UnitTestCase {
 	/**
 	 * @ticket 5809
 	 */
-	public function test_wp_update_term_duplicate_slug_different_taxonomy() {
+	public function test_wp_update_term_should_allow_duplicate_slugs_in_different_taxonomy() {
 		register_taxonomy( 'wptests_tax', 'post' );
 		register_taxonomy( 'wptests_tax_2', 'post' );
 
@@ -686,8 +686,78 @@ class Tests_Term extends WP_UnitTestCase {
 			'slug' => 'foo',
 		) );
 
-		$this->assertWPError( $updated );
-		$this->assertSame( 'duplicate_term_slug', $updated->get_error_code() );
+		$this->assertFalse( is_wp_error( $updated ) );
+
+		$t1_term = get_term( $t1, 'wptests_tax' );
+		$t2_term = get_term( $t2, 'wptests_tax_2' );
+		$this->assertSame( $t1_term->slug, $t2_term->slug );
+	}
+
+	/**
+	 * @ticket 30780
+	 */
+	public function test_wp_update_term_should_allow_duplicate_names_in_different_taxonomies() {
+		register_taxonomy( 'wptests_tax', 'post' );
+		register_taxonomy( 'wptests_tax_2', 'post' );
+
+		$t1 = $this->factory->term->create( array(
+			'name' => 'Foo',
+			'slug' => 'foo',
+			'taxonomy' => 'wptests_tax',
+		) );
+
+		$t2 = $this->factory->term->create( array(
+			'name' => 'Bar',
+			'slug' => 'bar',
+			'taxonomy' => 'wptests_tax_2',
+		) );
+
+		$updated = wp_update_term( $t2, 'wptests_tax_2', array(
+			'name' => 'Foo',
+		) );
+
+		$this->assertFalse( is_wp_error( $updated ) );
+
+		$t2_term = get_term( $t2, 'wptests_tax_2' );
+		$this->assertSame( 'Foo', $t2_term->name );
+	}
+
+	/**
+	 * @ticket 30780
+	 */
+	public function test_wp_update_term_should_allow_duplicate_names_at_different_levels_of_the_same_taxonomy() {
+		register_taxonomy( 'wptests_tax', 'post', array(
+			'hierarchical' => true,
+		) );
+
+		$t1 = $this->factory->term->create( array(
+			'name' => 'Foo',
+			'slug' => 'foo',
+			'taxonomy' => 'wptests_tax',
+		) );
+
+		$t2 = $this->factory->term->create( array(
+			'name' => 'Bar',
+			'slug' => 'bar',
+			'taxonomy' => 'wptests_tax',
+			'parent' => $t1,
+		) );
+
+		$t3 = $this->factory->term->create( array(
+			'name' => 'Bar Child',
+			'slug' => 'bar-child',
+			'taxonomy' => 'wptests_tax',
+			'parent' => $t2,
+		) );
+
+		$updated = wp_update_term( $t3, 'wptests_tax', array(
+			'name' => 'Bar',
+		) );
+
+		$this->assertFalse( is_wp_error( $updated ) );
+
+		$t3_term = get_term( $t3, 'wptests_tax' );
+		$this->assertSame( 'Bar', $t3_term->name );
 	}
 
 	public function test_wp_update_term_alias_of_no_term_group() {
@@ -1605,6 +1675,70 @@ class Tests_Term extends WP_UnitTestCase {
 		$this->assertWPError( $cat_id2 );
 	}
 
+	public function test_wp_update_term_should_assign_new_slug_when_reassigning_parent_as_long_as_there_is_no_other_term_with_the_same_slug() {
+		register_taxonomy( 'wptests_tax', 'post', array(
+			'hierarchical' => true,
+		) );
+		register_taxonomy( 'wptests_tax_2', 'post', array(
+			'hierarchical' => true,
+		) );
+
+		$t1 = $this->factory->term->create( array(
+			'taxonomy' => 'wptests_tax',
+			'slug' => 'parent-term',
+		) );
+
+		$t2 = $this->factory->term->create( array(
+			'taxonomy' => 'wptests_tax',
+			'slug' => 'foo',
+		) );
+
+		wp_update_term( $t2, 'wptests_tax', array(
+			'parent' => $t1,
+		) );
+
+		$t2_term = get_term( $t2, 'wptests_tax' );
+
+		$this->assertSame( 'foo', $t2_term->slug );
+
+		_unregister_taxonomy( 'wptests_tax' );
+	}
+
+	public function test_wp_update_term_should_not_assign_new_slug_when_reassigning_parent_as_long_as_there_is_no_other_slug_conflict_within_the_taxonomy() {
+		register_taxonomy( 'wptests_tax', 'post', array(
+			'hierarchical' => true,
+		) );
+		register_taxonomy( 'wptests_tax_2', 'post', array(
+			'hierarchical' => true,
+		) );
+
+		$t1 = $this->factory->term->create( array(
+			'taxonomy' => 'wptests_tax',
+			'slug' => 'parent-term',
+		) );
+
+		// Same slug but in a different tax.
+		$t2 = $this->factory->term->create( array(
+			'taxonomy' => 'wptests_tax_2',
+			'slug' => 'foo',
+		) );
+
+		$t3 = $this->factory->term->create( array(
+			'taxonomy' => 'wptests_tax',
+			'slug' => 'foo',
+		) );
+
+		wp_update_term( $t3, 'wptests_tax', array(
+			'parent' => $t1,
+		) );
+
+		$t3_term = get_term( $t3, 'wptests_tax' );
+
+		$this->assertSame( 'foo', $t3_term->slug );
+
+		_unregister_taxonomy( 'wptests_tax' );
+	}
+
 	/** Helpers **********************************************************/
 
 	public function _pre_insert_term_callback() {