diff --git a/phpunit/tests/rewrite-urls.php b/phpunit/tests/rewrite-urls.php
new file mode 100644
index 00000000..7a7360be
--- /dev/null
+++ b/phpunit/tests/rewrite-urls.php
@@ -0,0 +1,94 @@
+<?php
+
+require_once __DIR__ . '/base.php';
+
+use function WordPress\DataLiberation\URL\wp_rewrite_urls;
+
+/**
+ * @group import
+ * @group rewrite-urls
+ */
+class Tests_Import_Rewrite_Urls extends WP_Import_UnitTestCase {
+
+	const FROM = 'https://legacy-blog.com';
+	const TO   = 'https://modern-webstore.org';
+
+	/**
+	 * Run block markup through the URL rewriter with a single mapping.
+	 *
+	 * @param string $block_markup Markup to rewrite.
+	 * @return string Rewritten markup.
+	 */
+	private function rewrite( $block_markup ) {
+		return wp_rewrite_urls(
+			array(
+				'block_markup' => $block_markup,
+				'url-mapping'  => array( self::FROM => self::TO ),
+				'base_url'     => self::FROM,
+			)
+		);
+	}
+
+	/**
+	 * Fragment-only references point within the document that contains them,
+	 * so there is no origin to migrate and nothing to rewrite.
+	 *
+	 * Rewriting them resolves the fragment against the base URL first, which
+	 * turns an in-page anchor into a link to a different page: `#section`
+	 * became `/#section` and a bare `#` became `/`, dropping the fragment
+	 * altogether. Blocks whose saved markup contains such an href then fail
+	 * validation after an import.
+	 *
+	 * @covers ::WordPress\DataLiberation\URL\wp_rewrite_urls
+	 *
+	 * @dataProvider data_fragment_only_urls
+	 *
+	 * @param string $block_markup Markup containing a fragment-only reference.
+	 */
+	public function test_does_not_rewrite_fragment_only_urls( $block_markup ) {
+		$this->assertSame(
+			$block_markup,
+			$this->rewrite( $block_markup ),
+			'Fragment-only references must survive a rewrite untouched.'
+		);
+	}
+
+	/**
+	 * Data provider for fragment-only references.
+	 *
+	 * @return array<string, array<int, string>>
+	 */
+	public function data_fragment_only_urls() {
+		return array(
+			'named fragment'                => array( '<a href="#section">Jump</a>' ),
+			'bare hash'                     => array( '<a href="#">Placeholder</a>' ),
+			'hyphenated fragment'           => array( '<a href="#add-to-calendar">iCal</a>' ),
+			'fragment in a block attribute' => array( '<!-- wp:button {"url":"#login"} -->' ),
+		);
+	}
+
+	/**
+	 * The guard is limited to references that are *only* a fragment. A real
+	 * URL keeps being rewritten, and keeps its fragment while doing so.
+	 *
+	 * @covers ::WordPress\DataLiberation\URL\wp_rewrite_urls
+	 */
+	public function test_still_rewrites_absolute_urls_carrying_a_fragment() {
+		$this->assertSame(
+			'<a href="' . self::TO . '/page#section">Deep link</a>',
+			$this->rewrite( '<a href="' . self::FROM . '/page#section">Deep link</a>' )
+		);
+	}
+
+	/**
+	 * Ordinary rewriting is unaffected.
+	 *
+	 * @covers ::WordPress\DataLiberation\URL\wp_rewrite_urls
+	 */
+	public function test_still_rewrites_absolute_urls() {
+		$this->assertSame(
+			'<a href="' . self::TO . '/page">Page</a>',
+			$this->rewrite( '<a href="' . self::FROM . '/page">Page</a>' )
+		);
+	}
+}
diff --git a/src/php-toolkit/DataLiberation/URL/functions.php b/src/php-toolkit/DataLiberation/URL/functions.php
index 18d793a2..0be93dc4 100644
--- a/src/php-toolkit/DataLiberation/URL/functions.php
+++ b/src/php-toolkit/DataLiberation/URL/functions.php
@@ -53,6 +53,19 @@ function wp_rewrite_urls( $options ) {
 
 	$p = new BlockMarkupUrlProcessor( $options['block_markup'], $options['base_url'] );
 	while ( $p->next_url() ) {
+		/*
+		 * Leave fragment-only references alone. A URL like `#section` points
+		 * within the document that contains it, so there is no origin to
+		 * migrate. Resolving it against the base URL makes it look like a
+		 * child of the site being imported from, and rewriting it then turns
+		 * an in-page anchor into a link somewhere else entirely:
+		 * `#section` becomes `/#section`, and a bare `#` becomes `/`.
+		 */
+		$raw_url = $p->get_raw_url();
+		if ( is_string( $raw_url ) && 0 === strpos( $raw_url, '#' ) ) {
+			continue;
+		}
+
 		$parsed_url = $p->get_parsed_url();
 		foreach ( $url_mapping as $mapping ) {
 			if ( is_child_url_of( $parsed_url, $mapping['from_url'] ) ) {