diff --git a/src/wp-includes/feed-atom-comments.php b/src/wp-includes/feed-atom-comments.php
index 1de77e391794d..c0dc67f811f69 100644
--- a/src/wp-includes/feed-atom-comments.php
+++ b/src/wp-includes/feed-atom-comments.php
@@ -14,14 +14,18 @@
 <feed
 	xmlns="http://www.w3.org/2005/Atom"
 	xml:lang="<?php bloginfo_rss( 'language' ); ?>"
-	xmlns:thr="http://purl.org/syndication/thread/1.0"
 	<?php
+		feed_namespaces( 'atom-comments' );
+
 		/** This action is documented in wp-includes/feed-atom.php */
 		do_action( 'atom_ns' );
 
 		/**
 		 * Fires inside the feed tag in the Atom comment feed.
 		 *
+		 * Consider using the `wp_feed_namespaces` filter instead, which
+		 * prevents duplicate `xmlns` attributes.
+		 *
 		 * @since 2.8.0
 		 */
 		do_action( 'atom_comments_ns' );
diff --git a/src/wp-includes/feed-atom.php b/src/wp-includes/feed-atom.php
index 7dec79c840d3d..1e7c4f45ab40d 100644
--- a/src/wp-includes/feed-atom.php
+++ b/src/wp-includes/feed-atom.php
@@ -20,12 +20,16 @@
 ?>
 <feed
 	xmlns="http://www.w3.org/2005/Atom"
-	xmlns:thr="http://purl.org/syndication/thread/1.0"
 	xml:lang="<?php bloginfo_rss( 'language' ); ?>"
 	<?php
+	feed_namespaces( 'atom' );
+
 	/**
 	 * Fires at end of the Atom feed root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which prevents
+	 * duplicate `xmlns` attributes.
+	 *
 	 * @since 2.0.0
 	 */
 	do_action( 'atom_ns' );
diff --git a/src/wp-includes/feed-rdf.php b/src/wp-includes/feed-rdf.php
index 39cd15f04a911..b77cc8bc68aac 100644
--- a/src/wp-includes/feed-rdf.php
+++ b/src/wp-includes/feed-rdf.php
@@ -15,15 +15,15 @@
 ?>
 <rdf:RDF
 	xmlns="http://purl.org/rss/1.0/"
-	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-	xmlns:dc="http://purl.org/dc/elements/1.1/"
-	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
-	xmlns:admin="http://webns.net/mvcb/"
-	xmlns:content="http://purl.org/rss/1.0/modules/content/"
 	<?php
+	feed_namespaces( 'rdf' );
+
 	/**
 	 * Fires at the end of the feed root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which prevents
+	 * duplicate `xmlns` attributes.
+	 *
 	 * @since 2.0.0
 	 */
 	do_action( 'rdf_ns' );
diff --git a/src/wp-includes/feed-rss2-comments.php b/src/wp-includes/feed-rss2-comments.php
index 85a4079fa0df5..45d53599d8fa3 100644
--- a/src/wp-includes/feed-rss2-comments.php
+++ b/src/wp-includes/feed-rss2-comments.php
@@ -13,11 +13,9 @@
 do_action( 'rss_tag_pre', 'rss2-comments' );
 ?>
 <rss version="2.0"
-	xmlns:content="http://purl.org/rss/1.0/modules/content/"
-	xmlns:dc="http://purl.org/dc/elements/1.1/"
-	xmlns:atom="http://www.w3.org/2005/Atom"
-	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
 	<?php
+	feed_namespaces( 'rss2-comments' );
+
 	/** This action is documented in wp-includes/feed-rss2.php */
 	do_action( 'rss2_ns' );
 	?>
@@ -26,6 +24,9 @@
 	/**
 	 * Fires at the end of the RSS root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which
+	 * prevents duplicate `xmlns` attributes.
+	 *
 	 * @since 2.8.0
 	 */
 	do_action( 'rss2_comments_ns' );
diff --git a/src/wp-includes/feed-rss2.php b/src/wp-includes/feed-rss2.php
index 8b30313e6d606..a34dc2d460b13 100644
--- a/src/wp-includes/feed-rss2.php
+++ b/src/wp-includes/feed-rss2.php
@@ -21,16 +21,15 @@
 do_action( 'rss_tag_pre', 'rss2' );
 ?>
 <rss version="2.0"
-	xmlns:content="http://purl.org/rss/1.0/modules/content/"
-	xmlns:wfw="http://wellformedweb.org/CommentAPI/"
-	xmlns:dc="http://purl.org/dc/elements/1.1/"
-	xmlns:atom="http://www.w3.org/2005/Atom"
-	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
-	xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
 	<?php
+	feed_namespaces( 'rss2' );
+
 	/**
 	 * Fires at the end of the RSS root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which prevents
+	 * duplicate `xmlns` attributes.
+	 *
 	 * @since 2.0.0
 	 */
 	do_action( 'rss2_ns' );
diff --git a/src/wp-includes/feed.php b/src/wp-includes/feed.php
index 821a3eb9be804..a5885ab83deaf 100644
--- a/src/wp-includes/feed.php
+++ b/src/wp-includes/feed.php
@@ -699,6 +699,126 @@ function self_link() {
 	echo esc_url( apply_filters( 'self_link', get_self_link() ) );
 }
 
+/**
+ * Retrieves the XML namespaces for the root element of a feed.
+ *
+ * Namespaces are keyed by their prefix, so the same prefix cannot be
+ * declared twice. The default namespaces of a feed type cannot be removed,
+ * as the bundled feed templates use them in their static markup.
+ *
+ * @since 7.2.0
+ *
+ * @param string $type Type of feed. Possible values include 'rss2', 'rss2-comments',
+ *                     'rdf', 'atom', and 'atom-comments'.
+ * @return array<string, string> Array of namespace URIs, keyed by their prefix.
+ * @phpstan-param 'rss2'|'rss2-comments'|'rdf'|'atom'|'atom-comments' $type
+ * @phpstan-return array<non-falsy-string, non-falsy-string>
+ */
+function get_feed_namespaces( string $type ): array {
+	$defaults = array();
+
+	switch ( $type ) {
+		case 'rss2':
+			$defaults = array(
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+				'wfw'     => 'http://wellformedweb.org/CommentAPI/',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'atom'    => 'http://www.w3.org/2005/Atom',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+				'slash'   => 'http://purl.org/rss/1.0/modules/slash/',
+			);
+			break;
+
+		case 'rss2-comments':
+			$defaults = array(
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'atom'    => 'http://www.w3.org/2005/Atom',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+			);
+			break;
+
+		case 'rdf':
+			$defaults = array(
+				'rdf'     => 'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+				'admin'   => 'http://webns.net/mvcb/',
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+			);
+			break;
+
+		case 'atom':
+		case 'atom-comments':
+			$defaults = array(
+				'thr' => 'http://purl.org/syndication/thread/1.0',
+			);
+			break;
+	}
+
+	/**
+	 * Filters the XML namespaces of a feed's root element.
+	 *
+	 * Namespaces are keyed by their prefix, which makes duplicate `xmlns`
+	 * attributes impossible.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @param array<string, string> $namespaces Array of namespace URIs, keyed by their prefix.
+	 * @param string                $type       Type of feed. Possible values include 'rss2',
+	 *                                          'rss2-comments', 'rdf', 'atom', and 'atom-comments'.
+	 */
+	$namespaces = apply_filters( 'wp_feed_namespaces', $defaults, $type );
+
+	if ( ! is_array( $namespaces ) ) {
+		$namespaces = array();
+	}
+
+	// The bundled feed templates use the default namespaces, so they cannot be removed.
+	$namespaces = array_merge( $namespaces, $defaults );
+
+	$sanitized = array();
+
+	foreach ( $namespaces as $prefix => $uri ) {
+		$prefix = (string) $prefix;
+
+		// Prefixes must be valid XML names, and the `xml` and `xmlns` prefixes are reserved.
+		if ( ! preg_match( '/^[\p{L}_][\p{L}\p{M}\p{N}._\-\x{B7}]*$/u', $prefix )
+			|| in_array( strtolower( $prefix ), array( 'xml', 'xmlns' ), true )
+		) {
+			continue;
+		}
+
+		if ( ! is_string( $uri ) || empty( $uri ) ) {
+			continue;
+		}
+
+		$sanitized[ $prefix ] = $uri;
+	}
+
+	return $sanitized;
+}
+
+/**
+ * Displays the XML namespaces for the root element of a feed.
+ *
+ * Plugins should add namespaces via the `wp_feed_namespaces` filter instead
+ * of the older `{$type}_ns` actions, as two action callbacks printing the
+ * same namespace produce a duplicate attribute, which is a well-formedness
+ * error in XML.
+ *
+ * @since 7.2.0
+ *
+ * @param string $type Type of feed. Possible values include 'rss2', 'rss2-comments',
+ *                     'rdf', 'atom', and 'atom-comments'.
+ * @phpstan-param 'rss2'|'rss2-comments'|'rdf'|'atom'|'atom-comments' $type
+ */
+function feed_namespaces( string $type ): void {
+	foreach ( get_feed_namespaces( $type ) as $prefix => $uri ) {
+		printf( "xmlns:%s=\"%s\"\n\t", $prefix, esc_url( $uri ) );
+	}
+}
+
 /**
  * Gets the UTC time of the most recently modified post from WP_Query.
  *
diff --git a/tests/phpunit/tests/feed/feedNamespaces.php b/tests/phpunit/tests/feed/feedNamespaces.php
new file mode 100644
index 0000000000000..a8d1020271180
--- /dev/null
+++ b/tests/phpunit/tests/feed/feedNamespaces.php
@@ -0,0 +1,228 @@
+<?php
+
+/**
+ * Tests for the get_feed_namespaces() and feed_namespaces() functions.
+ *
+ * @group feed
+ *
+ * @covers ::get_feed_namespaces
+ * @covers ::feed_namespaces
+ */
+class Tests_Feed_FeedNamespaces extends WP_UnitTestCase {
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_should_return_default_rss2_namespaces() {
+		$namespaces = get_feed_namespaces( 'rss2' );
+
+		$this->assertSameSetsWithIndex(
+			array(
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+				'wfw'     => 'http://wellformedweb.org/CommentAPI/',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'atom'    => 'http://www.w3.org/2005/Atom',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+				'slash'   => 'http://purl.org/rss/1.0/modules/slash/',
+			),
+			$namespaces
+		);
+	}
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_should_return_default_atom_namespaces() {
+		$namespaces = get_feed_namespaces( 'atom' );
+
+		$this->assertSameSetsWithIndex(
+			array(
+				'thr' => 'http://purl.org/syndication/thread/1.0',
+			),
+			$namespaces
+		);
+	}
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_should_return_namespace_added_via_filter() {
+		add_filter(
+			'wp_feed_namespaces',
+			static function ( array $namespaces ): array {
+				$namespaces['source'] = 'http://source.scripting.com/';
+				return $namespaces;
+			}
+		);
+
+		$namespaces = get_feed_namespaces( 'rss2' );
+
+		$this->assertArrayHasKey( 'source', $namespaces );
+		$this->assertSame( 'http://source.scripting.com/', $namespaces['source'] );
+	}
+
+	/**
+	 * The feed type is passed to the filter, so namespaces can be added to
+	 * specific feeds only.
+	 *
+	 * @ticket 65785
+	 */
+	public function test_filter_should_receive_the_feed_type() {
+		add_filter(
+			'wp_feed_namespaces',
+			static function ( array $namespaces, string $type ): array {
+				if ( 'rss2' === $type ) {
+					$namespaces['source'] = 'http://source.scripting.com/';
+				}
+				return $namespaces;
+			},
+			10,
+			2
+		);
+
+		$this->assertArrayHasKey( 'source', get_feed_namespaces( 'rss2' ) );
+		$this->assertArrayNotHasKey( 'source', get_feed_namespaces( 'atom' ) );
+	}
+
+	/**
+	 * The bundled feed templates use the default namespaces in their static
+	 * markup, so a filter must not be able to remove them.
+	 *
+	 * @ticket 65785
+	 */
+	public function test_should_not_allow_removing_default_namespaces() {
+		add_filter(
+			'wp_feed_namespaces',
+			static function () {
+				return array( 'media' => 'http://search.yahoo.com/mrss/' );
+			}
+		);
+
+		$namespaces = get_feed_namespaces( 'rss2' );
+
+		$this->assertArrayHasKey( 'media', $namespaces );
+		$this->assertArrayHasKey( 'content', $namespaces );
+		$this->assertArrayHasKey( 'atom', $namespaces );
+	}
+
+	/**
+	 * A filter callback without a return value must not break the feed.
+	 *
+	 * @ticket 65785
+	 */
+	public function test_should_handle_a_non_array_filter_return() {
+		add_filter( 'wp_feed_namespaces', '__return_null' );
+
+		$namespaces = get_feed_namespaces( 'rss2' );
+
+		$this->assertArrayHasKey( 'content', $namespaces );
+	}
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_should_skip_invalid_prefixes() {
+		add_filter(
+			'wp_feed_namespaces',
+			static function ( array $namespaces ): array {
+				$namespaces['']            = 'http://example.org/empty';
+				$namespaces['foo bar']     = 'http://example.org/space';
+				$namespaces['"onload="x"'] = 'http://example.org/attack';
+				$namespaces['0numeric']    = 'http://example.org/numeric';
+				$namespaces['xmlns']       = 'http://example.org/reserved';
+				$namespaces['XML']         = 'http://example.org/reserved-too';
+				$namespaces['empty-uri']   = '';
+				return $namespaces;
+			}
+		);
+
+		$namespaces = get_feed_namespaces( 'rss2' );
+
+		$this->assertArrayNotHasKey( '', $namespaces );
+		$this->assertArrayNotHasKey( 'foo bar', $namespaces );
+		$this->assertArrayNotHasKey( '"onload="x"', $namespaces );
+		$this->assertArrayNotHasKey( '0numeric', $namespaces );
+		$this->assertArrayNotHasKey( 'xmlns', $namespaces );
+		$this->assertArrayNotHasKey( 'XML', $namespaces );
+		$this->assertArrayNotHasKey( 'empty-uri', $namespaces );
+	}
+
+	/**
+	 * Prefixes are XML names: mixed case like `creativeCommons` and
+	 * non-ASCII letters are valid and must be preserved.
+	 *
+	 * @ticket 65785
+	 */
+	public function test_should_preserve_valid_prefixes() {
+		add_filter(
+			'wp_feed_namespaces',
+			static function ( array $namespaces ): array {
+				$namespaces['creativeCommons'] = 'http://backend.userland.com/creativeCommonsRssModule';
+				$namespaces['média']           = 'http://example.org/media';
+				$namespaces['tag-uri']         = 'tag:example.org,2004:ns';
+				return $namespaces;
+			}
+		);
+
+		$namespaces = get_feed_namespaces( 'rss2' );
+
+		$this->assertArrayHasKey( 'creativeCommons', $namespaces );
+		$this->assertArrayHasKey( 'média', $namespaces );
+		// Namespace URIs are identifiers, not links, so non-http schemes must survive.
+		$this->assertSame( 'tag:example.org,2004:ns', $namespaces['tag-uri'] );
+	}
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_should_return_an_empty_array_for_an_unknown_type() {
+		$this->assertSame( array(), get_feed_namespaces( 'unknown' ) ); // @phpstan-ignore argument.type (Intentionally passing unsupported type.)
+	}
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_feed_namespaces_should_print_default_namespaces() {
+		$output = get_echo( 'feed_namespaces', array( 'rss2' ) );
+
+		$this->assertStringContainsString( 'xmlns:content="http://purl.org/rss/1.0/modules/content/"', $output );
+		$this->assertStringContainsString( 'xmlns:slash="http://purl.org/rss/1.0/modules/slash/"', $output );
+	}
+
+	/**
+	 * @ticket 65785
+	 */
+	public function test_feed_namespaces_should_escape_the_namespace_uri() {
+		add_filter(
+			'wp_feed_namespaces',
+			static function ( array $namespaces ): array {
+				$namespaces['evil'] = 'http://example.org/"><script>';
+				return $namespaces;
+			}
+		);
+
+		$output = get_echo( 'feed_namespaces', array( 'rss2' ) );
+
+		$this->assertStringNotContainsString( '"><script>', $output );
+		$this->assertStringContainsString( 'xmlns:evil=', $output );
+	}
+
+	/**
+	 * Two callbacks adding the same prefix must not result in a duplicate
+	 * attribute.
+	 *
+	 * @ticket 65785
+	 */
+	public function test_feed_namespaces_should_not_print_duplicate_prefixes() {
+		$add_source_ns = static function ( array $namespaces ): array {
+			$namespaces['source'] = 'http://source.scripting.com/';
+			return $namespaces;
+		};
+		add_filter( 'wp_feed_namespaces', $add_source_ns, 10 );
+		add_filter( 'wp_feed_namespaces', $add_source_ns, 11 );
+
+		$output = get_echo( 'feed_namespaces', array( 'rss2' ) );
+
+		$this->assertSame( 1, substr_count( $output, 'xmlns:source=' ) );
+	}
+}
diff --git a/tests/phpunit/tests/feed/rss2.php b/tests/phpunit/tests/feed/rss2.php
index c4f8807d7b7d3..94abc9d179739 100644
--- a/tests/phpunit/tests/feed/rss2.php
+++ b/tests/phpunit/tests/feed/rss2.php
@@ -90,6 +90,8 @@ public static function wpTearDownAfterClass() {
 
 	/**
 	 * This is a bit of a hack used to buffer feed content.
+	 *
+	 * @return non-falsy-string
 	 */
 	private function do_rss2() {
 		ob_start();
@@ -130,6 +132,41 @@ public function test_rss_element() {
 		$this->assertCount( 1, $rss[0]['child'] );
 	}
 
+	/**
+	 * Two plugins adding the same namespace via the `wp_feed_namespaces`
+	 * filter must not produce a duplicate attribute, which would be an XML
+	 * well-formedness error. The same goes for a plugin adding one of the
+	 * default namespaces of the feed.
+	 *
+	 * @ticket 65785
+	 */
+	public function test_wp_feed_namespaces_should_prevent_duplicate_namespaces() {
+		$add_source_ns = static function ( array $namespaces ): array {
+			$namespaces['source'] = 'http://source.scripting.com/';
+			$namespaces['atom']   = 'http://www.w3.org/2005/Atom';
+			return $namespaces;
+		};
+		add_filter( 'wp_feed_namespaces', $add_source_ns, 10 );
+		add_filter( 'wp_feed_namespaces', $add_source_ns, 11 );
+
+		$this->go_to( '/?feed=rss2' );
+		$feed = $this->do_rss2();
+
+		$this->assertSame( 1, substr_count( $feed, 'xmlns:source=' ) );
+		$this->assertSame( 1, substr_count( $feed, 'xmlns:atom=' ) );
+
+		// The feed must still parse: xml_to_array() returns an empty array on a parse error.
+		$xml = xml_to_array( $feed );
+		$rss = xml_find( $xml, 'rss' );
+
+		$this->assertCount( 1, $rss );
+		$this->assertIsArray( $rss[0] );
+		$this->assertArrayHasKey( 'attributes', $rss[0] );
+		$this->assertIsArray( $rss[0]['attributes'] );
+		$this->assertArrayHasKey( 'xmlns:source', $rss[0]['attributes'] );
+		$this->assertSame( 'http://source.scripting.com/', $rss[0]['attributes']['xmlns:source'] );
+	}
+
 	/**
 	 * [test_channel_element description]
 	 *