diff --git a/Lib/test/test_capi/test_codecs.py b/Lib/test/test_capi/test_codecs.py
new file mode 100644
index 00000000000000..e46726192aa05b
--- /dev/null
+++ b/Lib/test/test_capi/test_codecs.py
@@ -0,0 +1,54 @@
+import unittest
+from test.support import import_helper
+
+_testcapi = import_helper.import_module('_testcapi')
+
+
+class CAPITest(unittest.TestCase):
+
+    def test_decodeutf8(self):
+        """Test PyUnicode_DecodeUTF8()"""
+        decodeutf8 = _testcapi.unicode_decodeutf8
+
+        for s in ['abc', '\xa1\xa2', '\u4f60\u597d', 'a\U0001f600']:
+            b = s.encode('utf-8')
+            self.assertEqual(decodeutf8(b), s)
+            self.assertEqual(decodeutf8(b, 'strict'), s)
+
+        self.assertRaises(UnicodeDecodeError, decodeutf8, b'\x80')
+        self.assertRaises(UnicodeDecodeError, decodeutf8, b'\xc0')
+        self.assertRaises(UnicodeDecodeError, decodeutf8, b'\xff')
+        self.assertRaises(UnicodeDecodeError, decodeutf8, b'a\xf0\x9f')
+        self.assertEqual(decodeutf8(b'a\xf0\x9f', 'replace'), 'a\ufffd')
+        self.assertEqual(decodeutf8(b'a\xf0\x9fb', 'replace'), 'a\ufffdb')
+
+        self.assertRaises(LookupError, decodeutf8, b'a\x80', 'foo')
+        # TODO: Test PyUnicode_DecodeUTF8() with NULL as data and
+        # negative size.
+
+    def test_decodeutf8stateful(self):
+        """Test PyUnicode_DecodeUTF8Stateful()"""
+        decodeutf8stateful = _testcapi.unicode_decodeutf8stateful
+
+        for s in ['abc', '\xa1\xa2', '\u4f60\u597d', 'a\U0001f600']:
+            b = s.encode('utf-8')
+            self.assertEqual(decodeutf8stateful(b), (s, len(b)))
+            self.assertEqual(decodeutf8stateful(b, 'strict'), (s, len(b)))
+
+        self.assertRaises(UnicodeDecodeError, decodeutf8stateful, b'\x80')
+        self.assertRaises(UnicodeDecodeError, decodeutf8stateful, b'\xc0')
+        self.assertRaises(UnicodeDecodeError, decodeutf8stateful, b'\xff')
+        self.assertEqual(decodeutf8stateful(b'a\xf0\x9f'), ('a', 1))
+        self.assertEqual(decodeutf8stateful(b'a\xf0\x9f', 'replace'), ('a', 1))
+        self.assertRaises(UnicodeDecodeError, decodeutf8stateful, b'a\xf0\x9fb')
+        self.assertEqual(decodeutf8stateful(b'a\xf0\x9fb', 'replace'), ('a\ufffdb', 4))
+
+        self.assertRaises(LookupError, decodeutf8stateful, b'a\x80', 'foo')
+        # TODO: Test PyUnicode_DecodeUTF8Stateful() with NULL as data and
+        # negative size.
+        # TODO: Test PyUnicode_DecodeUTF8Stateful() with NULL as the address of
+        # "consumed".
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Misc/NEWS.d/next/C API/2022-11-20-09-52-50.gh-issue-99612.eBHksg.rst b/Misc/NEWS.d/next/C API/2022-11-20-09-52-50.gh-issue-99612.eBHksg.rst
new file mode 100644
index 00000000000000..40e3c8db5403c7
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2022-11-20-09-52-50.gh-issue-99612.eBHksg.rst	
@@ -0,0 +1,2 @@
+Fix :c:func:`PyUnicode_DecodeUTF8Stateful` for ASCII-only data:
+``*consumed`` was not set.
diff --git a/Modules/_testcapi/unicode.c b/Modules/_testcapi/unicode.c
index 4c5049dd406a7c..2d23993ce420b3 100644
--- a/Modules/_testcapi/unicode.c
+++ b/Modules/_testcapi/unicode.c
@@ -239,6 +239,40 @@ unicode_asutf8andsize(PyObject *self, PyObject *args)
     return Py_BuildValue("(Nn)", result, utf8_len);
 }
 
+/* Test PyUnicode_DecodeUTF8() */
+static PyObject *
+unicode_decodeutf8(PyObject *self, PyObject *args)
+{
+    const char *data;
+    Py_ssize_t size;
+    const char *errors = NULL;
+
+    if (!PyArg_ParseTuple(args, "y#|z", &data, &size, &errors))
+        return NULL;
+
+    return PyUnicode_DecodeUTF8(data, size, errors);
+}
+
+/* Test PyUnicode_DecodeUTF8Stateful() */
+static PyObject *
+unicode_decodeutf8stateful(PyObject *self, PyObject *args)
+{
+    const char *data;
+    Py_ssize_t size;
+    const char *errors = NULL;
+    Py_ssize_t consumed = 123456789;
+    PyObject *result;
+
+    if (!PyArg_ParseTuple(args, "y#|z", &data, &size, &errors))
+        return NULL;
+
+    result = PyUnicode_DecodeUTF8Stateful(data, size, errors, &consumed);
+    if (!result) {
+        return NULL;
+    }
+    return Py_BuildValue("(Nn)", result, consumed);
+}
+
 /* Test PyUnicode_Concat() */
 static PyObject *
 unicode_concat(PyObject *self, PyObject *args)
@@ -1025,6 +1059,8 @@ static PyMethodDef TestMethods[] = {
     {"unicode_asucs4",           unicode_asucs4,                 METH_VARARGS},
     {"unicode_asutf8",           unicode_asutf8,                 METH_VARARGS},
     {"unicode_asutf8andsize",    unicode_asutf8andsize,          METH_VARARGS},
+    {"unicode_decodeutf8",       unicode_decodeutf8,             METH_VARARGS},
+    {"unicode_decodeutf8stateful",unicode_decodeutf8stateful,    METH_VARARGS},
     {"unicode_concat",           unicode_concat,                 METH_VARARGS},
     {"unicode_splitlines",       unicode_splitlines,             METH_VARARGS},
     {"unicode_split",            unicode_split,                  METH_VARARGS},
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 55f029dd504ca0..19bde13a6f238a 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -4530,6 +4530,9 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
     }
     s += ascii_decode(s, end, PyUnicode_1BYTE_DATA(u));
     if (s == end) {
+        if (consumed) {
+            *consumed = size;
+        }
         return u;
     }