diff --git a/src/wp-includes/class-wp.php b/src/wp-includes/class-wp.php
index 84111bca9f..6d008c87da 100644
--- a/src/wp-includes/class-wp.php
+++ b/src/wp-includes/class-wp.php
@@ -300,6 +300,21 @@ class WP {
 			if ( $t->query_var && isset( $this->query_vars[$t->query_var] ) )
 				$this->query_vars[$t->query_var] = str_replace( ' ', '+', $this->query_vars[$t->query_var] );
 
+		// Don't allow non-public taxonomies to be queried from the front-end.
+		if ( ! is_admin() ) {
+			foreach ( get_taxonomies( array( 'public' => false ), 'objects' ) as $taxonomy => $t ) {
+				// Check first for taxonomy-specific query_var.
+				if ( $t->query_var && isset( $this->query_vars[ $t->query_var ] ) ) {
+					unset( $this->query_vars[ $t->query_var ] );
+				}
+
+				// Next, check the 'taxonomy' query_var.
+				if ( isset( $this->query_vars['taxonomy'] ) && $taxonomy === $this->query_vars['taxonomy'] ) {
+					unset( $this->query_vars['taxonomy'], $this->query_vars['term'] );
+				}
+			}
+		}
+
 		// Limit publicly queried post_types to those that are publicly_queryable
 		if ( isset( $this->query_vars['post_type']) ) {
 			$queryable_post_types = get_post_types( array('publicly_queryable' => true) );
diff --git a/src/wp-includes/taxonomy-functions.php b/src/wp-includes/taxonomy-functions.php
index eee37bc8a3..280f440775 100644
--- a/src/wp-includes/taxonomy-functions.php
+++ b/src/wp-includes/taxonomy-functions.php
@@ -278,7 +278,7 @@ function is_taxonomy_hierarchical($taxonomy) {
  *     * By default tag labels are used for non-hierarchical types and category labels for hierarchical ones.
  *     * You can see accepted values in {@link get_taxonomy_labels()}.
  * - description - A short descriptive summary of what the taxonomy is for. Defaults to blank.
- * - public - If the taxonomy should be publicly queryable; //@TODO not implemented.
+ * - public - If the taxonomy should be publicly queryable.
  *     * Defaults to true.
  * - hierarchical - Whether the taxonomy is hierarchical (e.g. category). Defaults to false.
  * - show_ui - Whether to generate a default UI for managing this taxonomy in the admin.
diff --git a/tests/phpunit/tests/taxonomy.php b/tests/phpunit/tests/taxonomy.php
index 8db5bfe31a..5a6af2e935 100644
--- a/tests/phpunit/tests/taxonomy.php
+++ b/tests/phpunit/tests/taxonomy.php
@@ -442,4 +442,44 @@ class Tests_Taxonomy extends WP_UnitTestCase {
 		$this->assertEqualSets( array( $t1 ), get_ancestors( $t2, 'wptests_conflict' ) );
 		_unregister_post_type( 'wptests_pt' );
 	}
+
+	/**
+	 * @ticket 21949
+	 */
+	public function test_nonpublic_taxonomy_should_not_be_queryable_using_taxname_query_var() {
+		register_taxonomy( 'wptests_tax', 'post', array(
+			'public' => false,
+		) );
+
+		$t = $this->factory->term->create_and_get( array(
+			'taxonomy' => 'wptests_tax',
+		) );
+
+		$p = $this->factory->post->create();
+		wp_set_object_terms( $p, $t->slug, 'wptests_tax' );
+
+		$this->go_to( '/?wptests_tax=' . $t->slug );
+
+		$this->assertFalse( is_tax( 'wptests_tax' ) );
+	}
+
+	/**
+	 * @ticket 21949
+	 */
+	public function test_nonpublic_taxonomy_should_not_be_queryable_using_taxonomy_and_term_vars() {
+		register_taxonomy( 'wptests_tax', 'post', array(
+			'public' => false,
+		) );
+
+		$t = $this->factory->term->create_and_get( array(
+			'taxonomy' => 'wptests_tax',
+		) );
+
+		$p = $this->factory->post->create();
+		wp_set_object_terms( $p, $t->slug, 'wptests_tax' );
+
+		$this->go_to( '/?taxonomy=wptests_tax&term=' . $t->slug );
+
+		$this->assertFalse( is_tax( 'wptests_tax' ) );
+	}
 }