diff --git a/docs/dom.md b/docs/dom.md
index 5ba90815..53c07d9f 100644
--- a/docs/dom.md
+++ b/docs/dom.md
@@ -1160,6 +1160,30 @@ $xml = $mapper($someNode);
 $xml = $doc->stringifyNode($someNode);
 ```
 
+#### to_unsafe_legacy_document
+
+Converts a `Dom\XMLDocument` (PHP 8.4+) into a legacy `DOMDocument` via an XML round-trip.
+This is useful when interoperating with libraries that still expect the legacy `DOMDocument` type.
+
+The `documentURI` is preserved on the resulting `DOMDocument`.
+
+**Caveat:** Line numbers in the resulting `DOMDocument` may differ from the original because
+the new DOM's `saveXML()` can reformat the output (e.g., collapsing multi-line opening tags
+into single lines).
+
+```php
+use VeeWee\Xml\Dom\Document;
+use function VeeWee\Xml\Dom\Mapper\to_unsafe_legacy_document;
+
+$doc = Document::fromXmlFile('some.xml');
+
+// Using the convenience method on Document:
+$legacyDoc = $doc->toUnsafeLegacyDocument();
+
+// Or using the mapper function directly:
+$legacyDoc = $doc->map(to_unsafe_legacy_document());
+```
+
 #### xslt_template
 
 Allows you to map an XML document based on an [XSLT template](xslt.md).
diff --git a/src/Xml/Dom/Document.php b/src/Xml/Dom/Document.php
index 38c35b1b..e6e88bc7 100644
--- a/src/Xml/Dom/Document.php
+++ b/src/Xml/Dom/Document.php
@@ -9,12 +9,14 @@
 use Dom\Node;
 use Dom\XMLDocument;
 use Dom\XPath as DOMXPath;
+use DOMDocument;
 use VeeWee\Xml\Dom\Traverser\Traverser;
 use VeeWee\Xml\Dom\Traverser\Visitor;
 use VeeWee\Xml\ErrorHandling\Issue\IssueCollection;
 use VeeWee\Xml\Exception\RuntimeException;
 use function Psl\Vec\map;
 use function VeeWee\Xml\Dom\Locator\document_element;
+use function VeeWee\Xml\Dom\Mapper\to_unsafe_legacy_document;
 use function VeeWee\Xml\Dom\Mapper\xml_string;
 use function VeeWee\Xml\Internal\configure;
 
@@ -101,6 +103,18 @@ public function toUnsafeDocument(): XMLDocument
         return $this->document;
     }
 
+    /**
+     * Converts this document into a legacy DOMDocument via an XML round-trip.
+     *
+     * The documentURI is preserved. Note that line numbers may differ from the original
+     * because the new DOM's saveXML() can reformat the output (e.g., collapsing
+     * multi-line opening tags into single lines).
+     */
+    public function toUnsafeLegacyDocument(): DOMDocument
+    {
+        return $this->map(to_unsafe_legacy_document());
+    }
+
     /**
      * @template T
      * @param callable(XMLDocument): T $locator
diff --git a/src/Xml/Dom/Mapper/to_unsafe_legacy_document.php b/src/Xml/Dom/Mapper/to_unsafe_legacy_document.php
new file mode 100644
index 00000000..fb01b203
--- /dev/null
+++ b/src/Xml/Dom/Mapper/to_unsafe_legacy_document.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace VeeWee\Xml\Dom\Mapper;
+
+use Closure;
+use Dom\XMLDocument;
+use DOMDocument;
+use function VeeWee\Xml\ErrorHandling\disallow_issues;
+use function VeeWee\Xml\ErrorHandling\disallow_libxml_false_returns;
+
+/**
+ * Converts a Dom\XMLDocument (PHP 8.4+) into a legacy DOMDocument.
+ *
+ * This performs an XML round-trip: the new DOM's saveXML() output is loaded
+ * into a legacy DOMDocument via loadXML(). The documentURI is preserved.
+ *
+ * Caveat: line numbers in the resulting DOMDocument may differ from the original
+ * because the new DOM's saveXML() can reformat the output (e.g., collapsing
+ * multi-line opening tags into single lines).
+ *
+ * @return Closure(XMLDocument): DOMDocument
+ */
+function to_unsafe_legacy_document(): Closure
+{
+    return static fn (XMLDocument $document): DOMDocument => disallow_issues(
+        static function () use ($document): DOMDocument {
+            $xml = xml_string()($document);
+
+            $legacy = new DOMDocument();
+            disallow_libxml_false_returns(
+                $legacy->loadXML($xml),
+                'Unable to load XML into legacy DOMDocument'
+            );
+
+            // documentURI must be set AFTER loadXML() because loadXML() resets it.
+            $documentUri = $document->documentURI;
+            if ($documentUri !== '') {
+                $legacy->documentURI = $documentUri;
+            }
+
+            return $legacy;
+        }
+    );
+}
diff --git a/src/bootstrap.php b/src/bootstrap.php
index 63f962f4..099050a4 100644
--- a/src/bootstrap.php
+++ b/src/bootstrap.php
@@ -71,6 +71,7 @@
         'Xml\Dom\Manipulator\Xmlns\rename' => __DIR__.'/Xml/Dom/Manipulator/Xmlns/rename.php',
         'Xml\Dom\Manipulator\Xmlns\rename_element_namespace' => __DIR__.'/Xml/Dom/Manipulator/Xmlns/rename_element_namespace.php',
         'Xml\Dom\Manipulator\append' => __DIR__.'/Xml/Dom/Manipulator/append.php',
+        'Xml\Dom\Mapper\to_unsafe_legacy_document' => __DIR__.'/Xml/Dom/Mapper/to_unsafe_legacy_document.php',
         'Xml\Dom\Mapper\xml_string' => __DIR__.'/Xml/Dom/Mapper/xml_string.php',
         'Xml\Dom\Mapper\xslt_template' => __DIR__.'/Xml/Dom/Mapper/xslt_template.php',
         'Xml\Dom\Predicate\is_attribute' => __DIR__.'/Xml/Dom/Predicate/is_attribute.php',
diff --git a/tests/Xml/Dom/Mapper/ToLegacyDocumentTest.php b/tests/Xml/Dom/Mapper/ToLegacyDocumentTest.php
new file mode 100644
index 00000000..bd801add
--- /dev/null
+++ b/tests/Xml/Dom/Mapper/ToLegacyDocumentTest.php
@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace VeeWee\Tests\Xml\Mapper;
+
+use DOMDocument;
+use PHPUnit\Framework\TestCase;
+use VeeWee\Xml\Dom\Document;
+use function VeeWee\Xml\Dom\Mapper\to_unsafe_legacy_document;
+
+final class ToLegacyDocumentTest extends TestCase
+{
+    public function test_it_can_convert_to_legacy_document(): void
+    {
+        $doc = Document::fromXmlString('<root><item>hello</item></root>');
+        $legacy = $doc->toUnsafeLegacyDocument();
+
+        static::assertInstanceOf(DOMDocument::class, $legacy);
+        static::assertSame('hello', $legacy->getElementsByTagName('item')->item(0)->textContent);
+    }
+
+    public function test_it_preserves_document_uri(): void
+    {
+        $doc = Document::fromXmlString(
+            '<root/>',
+            \VeeWee\Xml\Dom\Configurator\document_uri('/some/path/file.xml')
+        );
+        $legacy = $doc->toUnsafeLegacyDocument();
+
+        static::assertSame('/some/path/file.xml', $legacy->documentURI);
+    }
+
+    public function test_it_preserves_namespaces(): void
+    {
+        $xml = '<root xmlns:ns="http://example.com"><ns:item>value</ns:item></root>';
+        $doc = Document::fromXmlString($xml);
+        $legacy = $doc->toUnsafeLegacyDocument();
+
+        $items = $legacy->getElementsByTagNameNS('http://example.com', 'item');
+        static::assertSame(1, $items->length);
+        static::assertSame('value', $items->item(0)->textContent);
+    }
+
+    public function test_it_can_be_used_as_mapper(): void
+    {
+        $doc = Document::fromXmlString('<root/>');
+        $legacy = $doc->map(to_unsafe_legacy_document());
+
+        static::assertInstanceOf(DOMDocument::class, $legacy);
+        static::assertSame('root', $legacy->documentElement->localName);
+    }
+
+    public function test_it_preserves_xml_declaration(): void
+    {
+        $xml = '<?xml version="1.0" encoding="UTF-8"?>' . PHP_EOL . '<root/>';
+        $doc = Document::fromXmlString($xml);
+        $legacy = $doc->toUnsafeLegacyDocument();
+
+        static::assertSame('1.0', $legacy->xmlVersion);
+        static::assertSame('UTF-8', $legacy->encoding);
+    }
+}