diff --git src/wp-includes/default-filters.php src/wp-includes/default-filters.php
index 21419f2..c28594f 100644
--- src/wp-includes/default-filters.php
+++ src/wp-includes/default-filters.php
@@ -306,6 +306,10 @@ add_filter( 'authenticate', 'wp_authenticate_spam_check',         99    );
 add_filter( 'determine_current_user', 'wp_validate_auth_cookie'          );
 add_filter( 'determine_current_user', 'wp_validate_logged_in_cookie', 20 );
 
+// Split term updates.
+add_action( 'split_shared_term', '_wp_check_split_default_terms',  10, 4 );
+add_action( 'split_shared_term', '_wp_check_split_terms_in_menus', 10, 4 );
+
 /**
  * Filters formerly mixed into wp-includes
  */
diff --git src/wp-includes/taxonomy.php src/wp-includes/taxonomy.php
index 991f595..38d0fe7 100644
--- src/wp-includes/taxonomy.php
+++ src/wp-includes/taxonomy.php
@@ -3376,6 +3376,12 @@ function wp_update_term( $term_id, $taxonomy, $args = array() ) {
 
 	$tt_id = $wpdb->get_var( $wpdb->prepare( "SELECT tt.term_taxonomy_id FROM $wpdb->term_taxonomy AS tt INNER JOIN $wpdb->terms AS t ON tt.term_id = t.term_id WHERE tt.taxonomy = %s AND t.term_id = %d", $taxonomy, $term_id) );
 
+	// Check whether this is a shared term that needs splitting.
+	$_term_id = _split_shared_term( $term_id, $tt_id );
+	if ( ! is_wp_error( $_term_id ) ) {
+		$term_id = $_term_id;
+	}
+
 	/**
 	 * Fires immediately before the given terms are edited.
 	 *
@@ -4016,6 +4022,185 @@ function _update_generic_term_count( $terms, $taxonomy ) {
 }
 
 /**
+ * Create a new term for a term_taxonomy item that currently shares its term.
+ *
+ * @since 4.2.0
+ * @access private
+ *
+ * @param int   $term_id          ID of the shared term.
+ * @param int   $term_taxonomy_id ID of the term taxonomy item to receive a new term.
+ * @param array $shared_tts       Sibling term taxonomies, used for busting caches.
+ * @return int  Term ID.
+ */
+function _split_shared_term( $term_id, $term_taxonomy_id ) {
+	global $wpdb;
+
+	// Don't try to split terms if database schema does not support shared slugs.
+	$current_db_version = get_option( 'db_version' );
+	if ( $current_db_version < 30133 ) {
+		return $term_id;
+	}
+
+	// If there are no shared term_taxonomy rows, there's nothing to do here.
+	$shared_tt_count = $wpdb->get_var( $wpdb->prepare( "SELECT COUNT(*) FROM $wpdb->term_taxonomy tt WHERE tt.term_id = %d AND tt.term_taxonomy_id != %d", $term_id, $term_taxonomy_id ) );
+	if ( ! $shared_tt_count ) {
+		return $term_id;
+	}
+
+	// Pull up data about the currently shared slug, which we'll use to populate the new one.
+	$shared_term = $wpdb->get_row( $wpdb->prepare( "SELECT t.* FROM $wpdb->terms t WHERE t.term_id = %d", $term_id ) );
+
+	$new_term_data = array(
+		'name' => $shared_term->name,
+		'slug' => $shared_term->slug,
+		'term_group' => $shared_term->term_group,
+	);
+
+	if ( false === $wpdb->insert( $wpdb->terms, $new_term_data ) ) {
+		return new WP_Error( 'db_insert_error', __( 'Could not split shared term.' ), $wpdb->last_error );
+	}
+
+	$new_term_id = (int) $wpdb->insert_id;
+
+	// Update the existing term_taxonomy to point to the newly created term.
+	$wpdb->update( $wpdb->term_taxonomy,
+		array( 'term_id' => $new_term_id ),
+		array( 'term_taxonomy_id' => $term_taxonomy_id )
+	);
+
+	// Reassign child terms to the new parent.
+	$term_taxonomy = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $wpdb->term_taxonomy WHERE term_taxonomy_id = %d", $term_taxonomy_id ) );
+	$children_tt_ids = $wpdb->get_col( $wpdb->prepare( "SELECT term_taxonomy_id FROM $wpdb->term_taxonomy WHERE taxonomy = %s AND parent = %d", $term_taxonomy->taxonomy, $term_id ) );
+
+	if ( ! empty( $children_tt_ids ) ) {
+		foreach ( $children_tt_ids as $child_tt_id ) {
+			$wpdb->update( $wpdb->term_taxonomy,
+				array( 'parent' => $new_term_id ),
+				array( 'term_taxonomy_id' => $child_tt_id )
+			);
+			clean_term_cache( $term_id, $term_taxonomy->taxonomy );
+		}
+	} else {
+		// If the term has no children, we must force its taxonomy cache to be rebuilt separately.
+		clean_term_cache( $new_term_id, $term_taxonomy->taxonomy );
+	}
+
+	// Clean the cache for term taxonomies formerly shared with the current term.
+	$shared_term_taxonomies = $wpdb->get_row( $wpdb->prepare( "SELECT taxonomy FROM $wpdb->term_taxonomy WHERE term_id = %d", $term_id ) );
+	foreach ( (array) $shared_term_taxonomies as $shared_term_taxonomy ) {
+		clean_term_cache( $term_id, $shared_term_taxonomy );
+	}
+
+	// Keep a record of term_ids that have been split, keyed by old term_id.
+	$split_term_data = get_option( '_split_terms', array() );
+	if ( ! isset( $split_term_data[ $term_id ] ) ) {
+		$split_term_data[ $term_id ] = array();
+	}
+
+	$split_term_data[ $term_id ][] = array(
+		'new_term_id'      => $new_term_id,
+		'term_taxonomy_id' => $term_taxonomy_id,
+		'taxonomy'         => $term_taxonomy->taxonomy,
+	);
+
+	update_option( '_split_terms', $split_term_data );
+
+	/**
+	 * Fires after a previously shared taxonomy term is split into two separate terms.
+	 *
+	 * @since 4.2.0
+	 *
+	 * @param int    $term_id          ID of the formerly shared term.
+	 * @param int    $new_term_id      ID of the new term created for the $term_taxonomy_id.
+	 * @param int    $term_taxonomy_id ID for the term_taxonomy row affected by the split.
+	 * @param string $taxonomy         Taxonomy for the split term.
+	 */
+	do_action( 'split_shared_term', $term_id, $new_term_id, $term_taxonomy_id, $term_taxonomy->taxonomy );
+
+	return $new_term_id;
+}
+
+/**
+ * Check default categories when a term gets split to see if any of them need
+ * to be updated.
+ *
+ * @since 4.2.0
+ * @access private
+ *
+ * @param int    $term_id          ID of the formerly shared term.
+ * @param int    $new_term_id      ID of the new term created for the $term_taxonomy_id.
+ * @param int    $term_taxonomy_id ID for the term_taxonomy row affected by the split.
+ * @param string $taxonomy         Taxonomy for the split term.
+ */
+function _wp_check_split_default_terms( $term_id, $new_term_id, $term_taxonomy_id, $taxonomy ) {
+	if ( 'category' == $taxonomy ) {
+		foreach ( array( 'default_category', 'default_link_category', 'default_email_category' ) as $option ) {
+			if ( $term_id == get_option( $option, -1 ) ) {
+				update_option( $option, $new_term_id );
+			}
+		}
+	}
+}
+
+/**
+ * Check menu items when a term gets split to see if any of them need to be
+ * updated.
+ *
+ * @since 4.2.0
+ * @access private
+ *
+ * @param int    $term_id          ID of the formerly shared term.
+ * @param int    $new_term_id      ID of the new term created for the $term_taxonomy_id.
+ * @param int    $term_taxonomy_id ID for the term_taxonomy row affected by the split.
+ * @param string $taxonomy         Taxonomy for the split term.
+ */
+function _wp_check_split_terms_in_menus( $term_id, $new_term_id, $term_taxonomy_id, $taxonomy ) {
+	global $wpdb;
+	$post_ids = $wpdb->get_col( $wpdb->prepare(
+		"SELECT m1.post_id
+		FROM {$wpdb->postmeta} AS m1
+			INNER JOIN {$wpdb->postmeta} AS m2 ON m2.post_id=m1.post_id
+			INNER JOIN {$wpdb->postmeta} AS m3 ON m3.post_id=m1.post_id
+		WHERE ( m1.meta_key = '_menu_item_type' AND m1.meta_value = 'taxonomy' )
+			AND ( m2.meta_key = '_menu_item_object' AND m2.meta_value = '%s' )
+			AND ( m3.meta_key = '_menu_item_object_id' AND m3.meta_value = %d )",
+		$taxonomy,
+		$term_id
+	) );
+
+	if ( $post_ids ) {
+		foreach ( $post_ids as $post_id ) {
+			update_post_meta( $post_id, '_menu_item_object_id', $new_term_id, $term_id );
+		}
+	}
+}
+
+/**
+ * Get data about a previously split taxonomy term.
+ *
+ * @since 4.2.0
+ *
+ * @param int $old_term_id Term ID. This is the old, pre-split term ID.
+ * @param string $taxonomy Optional. If provided, results will be limited to those matching the given taxonomy.
+ * @return array Array of data about matching terms that have been previously split. Each set of term data contains
+ *               the 'new_term_id', the 'term_taxonomy_id', and the 'taxonomy' of the split term.
+ */
+function wp_get_split_terms( $old_term_id, $taxonomy = '' ) {
+	$split_term_data = get_option( '_split_terms', array() );
+
+	$terms = array();
+	if ( isset( $split_term_data[ $old_term_id ] ) ) {
+		$terms = $split_term_data[ $old_term_id ];
+	}
+
+	if ( $taxonomy ) {
+		$terms = wp_list_filter( $terms, array( 'taxonomy' => $taxonomy ) );
+	}
+
+	return $terms;
+}
+
+/**
  * Generate a permalink for a taxonomy term archive.
  *
  * @since 2.5.0
diff --git tests/phpunit/tests/term.php tests/phpunit/tests/term.php
index d717bf6..0237ac9 100644
--- tests/phpunit/tests/term.php
+++ tests/phpunit/tests/term.php
@@ -760,6 +760,88 @@ class Tests_Term extends WP_UnitTestCase {
 		$this->assertSame( 'Bar', $t3_term->name );
 	}
 
+	/**
+	 * @ticket 5809
+	 */
+	public function test_wp_update_term_should_split_shared_term() {
+		global $wpdb;
+
+		register_taxonomy( 'wptests_tax', 'post' );
+		register_taxonomy( 'wptests_tax_2', 'post' );
+
+		$t1 = wp_insert_term( 'Foo', 'wptests_tax' );
+		$t2 = wp_insert_term( 'Foo', 'wptests_tax_2' );
+
+		// Manually modify because split terms shouldn't naturally occur.
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t2['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+
+		$posts = $this->factory->post->create_many( 2 );
+		wp_set_object_terms( $posts[0], array( 'Foo' ), 'wptests_tax' );
+		wp_set_object_terms( $posts[1], array( 'Foo' ), 'wptests_tax_2' );
+
+		// Verify that the terms are shared.
+		$t1_terms = wp_get_object_terms( $posts[0], 'wptests_tax' );
+		$t2_terms = wp_get_object_terms( $posts[1], 'wptests_tax_2' );
+		$this->assertSame( $t1_terms[0]->term_id, $t2_terms[0]->term_id );
+
+		wp_update_term( $t2_terms[0]->term_id, 'wptests_tax_2', array(
+			'name' => 'New Foo',
+		) );
+
+		$t1_terms = wp_get_object_terms( $posts[0], 'wptests_tax' );
+		$t2_terms = wp_get_object_terms( $posts[1], 'wptests_tax_2' );
+		$this->assertNotEquals( $t1_terms[0]->term_id, $t2_terms[0]->term_id );
+	}
+
+	/**
+	 * @ticket 5809
+	 */
+	public function test_wp_update_term_should_not_split_shared_term_before_410_schema_change() {
+		global $wpdb;
+
+		$db_version = get_option( 'db_version' );
+		update_option( 'db_version', 30055 );
+
+		register_taxonomy( 'wptests_tax', 'post' );
+		register_taxonomy( 'wptests_tax_2', 'post' );
+
+		$t1 = wp_insert_term( 'Foo', 'wptests_tax' );
+		$t2 = wp_insert_term( 'Foo', 'wptests_tax_2' );
+
+		// Manually modify because split terms shouldn't naturally occur.
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t2['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+
+		$posts = $this->factory->post->create_many( 2 );
+		wp_set_object_terms( $posts[0], array( 'Foo' ), 'wptests_tax' );
+		wp_set_object_terms( $posts[1], array( 'Foo' ), 'wptests_tax_2' );
+
+		// Verify that the term is shared.
+		$t1_terms = wp_get_object_terms( $posts[0], 'wptests_tax' );
+		$t2_terms = wp_get_object_terms( $posts[1], 'wptests_tax_2' );
+		$this->assertSame( $t1_terms[0]->term_id, $t2_terms[0]->term_id );
+
+		wp_update_term( $t2_terms[0]->term_id, 'wptests_tax_2', array(
+			'name' => 'New Foo',
+		) );
+
+		// Term should still be shared.
+		$t1_terms = wp_get_object_terms( $posts[0], 'wptests_tax' );
+		$t2_terms = wp_get_object_terms( $posts[1], 'wptests_tax_2' );
+		$this->assertSame( $t1_terms[0]->term_id, $t2_terms[0]->term_id );
+
+		update_option( 'db_version', $db_version );
+	}
+
 	public function test_wp_update_term_alias_of_no_term_group() {
 		register_taxonomy( 'wptests_tax', 'post' );
 		$t1 = $this->factory->term->create( array(
diff --git tests/phpunit/tests/term/splitSharedTerm.php tests/phpunit/tests/term/splitSharedTerm.php
new file mode 100644
index 0000000..d755e7b
--- /dev/null
+++ tests/phpunit/tests/term/splitSharedTerm.php
@@ -0,0 +1,206 @@
+<?php
+
+/**
+ * @group taxonomy
+ */
+class Tests_Term_SplitSharedTerm extends WP_UnitTestCase {
+	protected $terms = array();
+
+	public function setUp() {
+		global $wpdb;
+
+		parent::setUp();
+
+		register_taxonomy( 'wptests_tax', 'post' );
+		register_taxonomy( 'wptests_tax_2', 'post', array(
+			'hierarchical' => true,
+		) );
+		register_taxonomy( 'wptests_tax_3', 'post' );
+
+		$t1 = wp_insert_term( 'Foo', 'wptests_tax' );
+		$t2 = wp_insert_term( 'Foo', 'wptests_tax_2' );
+		$t3 = wp_insert_term( 'Foo', 'wptests_tax_3' );
+
+		// Manually modify because split terms shouldn't naturally occur.
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t2['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t3['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+
+		$t2_child = wp_insert_term( 'Foo Child', 'wptests_tax_2', array(
+			'parent' => $t1['term_id'],
+		) );
+
+		$this->terms = array(
+			't1' => $t1,
+			't2' => $t2,
+			't3' => $t3,
+			't2_child' => $t2_child,
+		);
+
+		_split_shared_term( $t1['term_id'], $t2['term_taxonomy_id'] );
+		_split_shared_term( $t1['term_id'], $t3['term_taxonomy_id'] );
+	}
+
+	/**
+	 * @ticket 5809
+	 */
+	public function test_should_create_new_term_ids() {
+		$t1_term = get_term_by( 'term_taxonomy_id', $this->terms['t1']['term_taxonomy_id'], 'wptests_tax' );
+		$t2_term = get_term_by( 'term_taxonomy_id', $this->terms['t2']['term_taxonomy_id'], 'wptests_tax_2' );
+		$t3_term = get_term_by( 'term_taxonomy_id', $this->terms['t3']['term_taxonomy_id'], 'wptests_tax_3' );
+
+		$this->assertNotEquals( $t1_term->term_id, $t2_term->term_id );
+		$this->assertNotEquals( $t1_term->term_id, $t3_term->term_id );
+		$this->assertNotEquals( $t2_term->term_id, $t3_term->term_id );
+	}
+
+	/**
+	 * @ticket 5809
+	 */
+	public function test_should_retain_child_terms_when_using_get_terms_parent() {
+		$t2_term = get_term_by( 'term_taxonomy_id', $this->terms['t2']['term_taxonomy_id'], 'wptests_tax_2' );
+		$children = get_terms( 'wptests_tax_2', array(
+			'parent' => $t2_term->term_id,
+			'hide_empty' => false,
+		) );
+
+		$this->assertEquals( $this->terms['t2_child']['term_taxonomy_id'], $children[0]->term_taxonomy_id );
+	}
+
+	/**
+	 * @ticket 5809
+	 */
+	public function test_should_retain_child_terms_when_using_get_terms_child_of() {
+		$t2_term = get_term_by( 'term_taxonomy_id', $this->terms['t2']['term_taxonomy_id'], 'wptests_tax_2' );
+		$children = get_terms( 'wptests_tax_2', array(
+			'child_of' => $t2_term->term_id,
+			'hide_empty' => false,
+		) );
+
+		$this->assertEquals( $this->terms['t2_child']['term_taxonomy_id'], $children[0]->term_taxonomy_id );
+	}
+
+	/**
+	 * @ticket 30335
+	 */
+	public function test_should_rebuild_split_term_taxonomy_hierarchy() {
+		global $wpdb;
+
+		register_taxonomy( 'wptests_tax_3', 'post' );
+		register_taxonomy( 'wptests_tax_4', 'post', array(
+			'hierarchical' => true,
+		) );
+
+		$t1 = wp_insert_term( 'Foo1', 'wptests_tax_3' );
+		$t2 = wp_insert_term( 'Foo1 Parent', 'wptests_tax_4' );
+		$t3 = wp_insert_term( 'Foo1', 'wptests_tax_4', array(
+			'parent' => $t2['term_id'],
+		) );
+
+		// Manually modify because split terms shouldn't naturally occur.
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t3['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+		$th = _get_term_hierarchy( 'wptests_tax_4' );
+
+		$new_term_id = _split_shared_term( $t1['term_id'], $t3['term_taxonomy_id'] );
+
+		$t2_children = get_term_children( $t2['term_id'], 'wptests_tax_4' );
+		$this->assertEquals( array( $new_term_id ), $t2_children );
+	}
+
+	/**
+	 * @ticket 30335
+	 */
+	public function test_should_update_default_category_on_term_split() {
+		global $wpdb;
+		$t1 = wp_insert_term( 'Foo Default', 'category' );
+
+		update_option( 'default_category', $t1['term_id'] );
+
+		register_taxonomy( 'wptests_tax_5', 'post' );
+		$t2 = wp_insert_term( 'Foo Default', 'wptests_tax_5' );
+
+		// Manually modify because split terms shouldn't naturally occur.
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t2['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+
+		$this->assertEquals( $t1['term_id'], get_option( 'default_category', -1 ) );
+
+		$new_term_id = _split_shared_term( $t1['term_id'], $t1['term_taxonomy_id'] );
+
+		$this->assertNotEquals( $new_term_id, $t1['term_id'] );
+		$this->assertEquals( $new_term_id, get_option( 'default_category', -1 ) );
+	}
+
+	/**
+	 * @ticket 30335
+	 */
+	public function test_should_update_menus_on_term_split() {
+		global $wpdb;
+
+		$t1 = wp_insert_term( 'Foo Menu', 'category' );
+
+		register_taxonomy( 'wptests_tax_6', 'post' );
+		$t2 = wp_insert_term( 'Foo Menu', 'wptests_tax_6' );
+
+		// Manually modify because split terms shouldn't naturally occur.
+		$wpdb->update( $wpdb->term_taxonomy,
+			array( 'term_id' => $t1['term_id'] ),
+			array( 'term_taxonomy_id' => $t2['term_taxonomy_id'] ),
+			array( '%d' ),
+			array( '%d' )
+		);
+
+		$menu_id = wp_create_nav_menu( rand_str() );
+		$cat_menu_item = wp_update_nav_menu_item( $menu_id, 0, array(
+			'menu-item-type' => 'taxonomy',
+			'menu-item-object' => 'category',
+			'menu-item-object-id' => $t1['term_id'],
+			'menu-item-status' => 'publish'
+		) );
+		$this->assertEquals( $t1['term_id'], get_post_meta( $cat_menu_item, '_menu_item_object_id', true ) );
+
+		$new_term_id = _split_shared_term( $t1['term_id'], $t1['term_taxonomy_id'] );
+		$this->assertNotEquals( $new_term_id, $t1['term_id'] );
+		$this->assertEquals( $new_term_id, get_post_meta( $cat_menu_item, '_menu_item_object_id', true ) );
+	}
+
+	public function test_wp_get_split_shared_term_with_no_taxonomy() {
+		$found = wp_get_split_terms( $this->terms['t1']['term_id'] );
+
+		$expected = array(
+			$this->terms['t2']['term_taxonomy_id'],
+			$this->terms['t3']['term_taxonomy_id'],
+		);
+
+		$this->assertEqualSets( $expected, wp_list_pluck( $found, 'term_taxonomy_id' ) );
+	}
+
+	public function test_wp_get_split_shared_term_with_taxonomy() {
+		$found = wp_get_split_terms( $this->terms['t1']['term_id'], 'wptests_tax_3' );
+
+		$expected = array(
+			$this->terms['t3']['term_taxonomy_id'],
+		);
+
+		$this->assertEqualSets( $expected, wp_list_pluck( $found, 'term_taxonomy_id' ) );
+	}
+}