diff --git a/Lib/test/test_pyexpat.py b/Lib/test/test_pyexpat.py
--- a/Lib/test/test_pyexpat.py
+++ b/Lib/test/test_pyexpat.py
@@ -641,6 +641,16 @@
         parser.Parse("<?xml version='1.0'?><element/>")
         self.assertEqual(handler_call_args, [(None, None)])
 
+        # test UseForeignDTD() is equal to UseForeignDTD(True)
+        handler_call_args[:] = []
+
+        parser = expat.ParserCreate()
+        parser.UseForeignDTD()
+        parser.SetParamEntityParsing(expat.XML_PARAM_ENTITY_PARSING_ALWAYS)
+        parser.ExternalEntityRefHandler = resolve_entity
+        parser.Parse("<?xml version='1.0'?><element/>")
+        self.assertEqual(handler_call_args, [(None, None)])
+
     def test_ignore_use_foreign_dtd(self):
         """
         If UseForeignDTD is passed True and a document with an external
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -71,6 +71,9 @@
 Extension Modules
 -----------------
 
+- Issue #16012: Fix a regression in pyexpat. The parser's UseForeignDTD()
+  method doesn't require an argument again.
+
 Tests
 -----
 
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -1035,7 +1035,7 @@
 {
     int flag = 1;
     enum XML_Error rc;
-    if (!PyArg_ParseTuple(args, "p:UseForeignDTD", &flag))
+    if (!PyArg_ParseTuple(args, "|p:UseForeignDTD", &flag))
         return NULL;
     rc = XML_UseForeignDTD(self->itself, flag ? XML_TRUE : XML_FALSE);
     if (rc != XML_ERROR_NONE) {