diff --git a/ext/dom/VmDom.php b/ext/dom/VmDom.php
index f6b55d93551..2396838d9bb 100644
--- a/ext/dom/VmDom.php
+++ b/ext/dom/VmDom.php
@@ -1044,6 +1044,9 @@ private static function indexElementIdsRecursive(ObjectEntry $document, ObjectEn
         if (null === $idAttr && null !== $nodeState->idAttributeName) {
             $idAttr = $nodeState->idAttributeName;
         }
+        if (null === $idAttr && $docState->isHtmlDocument && isset($nodeState->attributes['id'])) {
+            $idAttr = 'id';
+        }
         if (null !== $idAttr) {
             $value = $nodeState->attributes[$idAttr] ?? null;
             if (null !== $value && '' !== $value) {
diff --git a/test/compliance/cases/dom/document_loadhtml_get_element_by_id.phpt b/test/compliance/cases/dom/document_loadhtml_get_element_by_id.phpt
new file mode 100644
index 00000000000..8c4bfa41dd5
--- /dev/null
+++ b/test/compliance/cases/dom/document_loadhtml_get_element_by_id.phpt
@@ -0,0 +1,12 @@
+--TEST--
+dom DOMDocument::getElementById() after loadHTML() HTML id attribute (#15251)
+--FILE--
+<?php
+$doc = new DOMDocument();
+$doc->loadHTML('<p id="target">hello</p>');
+$found = $doc->getElementById('target');
+echo null !== $found ? $found->textContent : 'null', "\n";
+echo null === $doc->getElementById('missing') ? 'missing_null' : 'missing_found', "\n";
+--EXPECT--
+hello
+missing_null
diff --git a/test/repro/maintainer_gap_dom_loadhtml_get_element_by_id.php b/test/repro/maintainer_gap_dom_loadhtml_get_element_by_id.php
new file mode 100644
index 00000000000..919b9dea6c7
--- /dev/null
+++ b/test/repro/maintainer_gap_dom_loadhtml_get_element_by_id.php
@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Issue #15251 — DOMDocument::getElementById() after loadHTML() indexes HTML id attributes.
+ */
+
+$doc = new DOMDocument();
+$doc->loadHTML('<p id="target">hello</p>');
+$found = $doc->getElementById('target');
+if (null === $found) {
+    echo "fail: getElementById returned null\n";
+    exit(1);
+}
+if ('hello' !== $found->textContent) {
+    echo 'fail: textContent=', $found->textContent, "\n";
+    exit(1);
+}
+if (null !== $doc->getElementById('missing')) {
+    echo "fail: missing id should be null\n";
+    exit(1);
+}
+
+echo "ok\n";