diff --git a/sphinx/domains/cpp.py b/sphinx/domains/cpp.py
index 2bcdf4432d3..d9ce70623e4 100644
--- a/sphinx/domains/cpp.py
+++ b/sphinx/domains/cpp.py
@@ -6983,7 +6983,7 @@ def parse_declaration(self, objectType: str, directiveType: str) -> ASTDeclarati
         if self.match(_visibility_re):
             visibility = self.matched_text
 
-        if objectType in ('type', 'concept', 'member', 'function', 'class'):
+        if objectType in ('type', 'concept', 'member', 'function', 'class', 'union'):
             templatePrefix = self._parse_template_declaration_prefix(objectType)
 
         if objectType == 'type':
diff --git a/tests/test_domain_cpp.py b/tests/test_domain_cpp.py
index 977426c8029..05a111e3999 100644
--- a/tests/test_domain_cpp.py
+++ b/tests/test_domain_cpp.py
@@ -917,6 +917,9 @@ def test_domain_cpp_ast_requires_clauses():
     check('class',
           'template<typename T> requires IsValid<T> {key}T : Base',
           {4: 'I0EIQ7IsValidI1TEE1T'}, key='class')
+    check('union',
+          'template<typename T> requires IsValid<T> {key}T',
+          {4: 'I0EIQ7IsValidI1TEE1T'}, key='union')
     check('member',
           'template<typename T> requires IsValid<T> int Val = 7',
           {4: 'I0EIQ7IsValidI1TEE3Val'})