diff --git a/sphinx/domains/cpp.py b/sphinx/domains/cpp.py
index fe52d881d3f..972cbb30593 100644
--- a/sphinx/domains/cpp.py
+++ b/sphinx/domains/cpp.py
@@ -10,7 +10,8 @@
 
 import re
 from typing import (
-    Any, Callable, Dict, Generator, Iterator, List, Tuple, Type, TypeVar, Union, Optional
+    Any, Callable, Dict, Generator, Iterator, List, Tuple, Type, TypeVar, Union, Optional,
+    cast
 )
 
 from docutils import nodes
@@ -298,6 +299,8 @@
 
 _string_re = re.compile(r"[LuU8]?('([^'\\]*(?:\\.[^'\\]*)*)'"
                         r'|"([^"\\]*(?:\\.[^"\\]*)*)")', re.S)
+_udl_suffix_re = re.compile(r"[A-Za-z_][A-Za-z0-9_]*")
+_int_suffix_re = re.compile(r"(?:[uU](?:[lL]{1,2})?|(?:[lL]{1,2})[uU]?)")
 _visibility_re = re.compile(r'\b(public|private|protected)\b')
 _operator_re = re.compile(r'''(?x)
         \[\s*\]
@@ -4662,8 +4665,21 @@ def _parse_literal(self) -> ASTLiteral:
                       integer_literal_re, octal_literal_re]:
             pos = self.pos
             if self.match(regex):
-                while self.current_char in 'uUlLfF':
-                    self.pos += 1
+                consumed_builtin = False
+                if regex is float_literal_re:
+                    if not self.eof and self.current_char in 'fFlL':
+                        consumed_builtin = True
+                        self.pos += 1
+                else:
+                    if not self.eof:
+                        builtin_match = _int_suffix_re.match(self.definition, self.pos)
+                        if builtin_match:
+                            self.pos = builtin_match.end()
+                            consumed_builtin = True
+                if not consumed_builtin:
+                    udl_match = _udl_suffix_re.match(self.definition, self.pos)
+                    if udl_match:
+                        self.pos = udl_match.end()
                 return ASTNumberLiteral(self.definition[pos:self.pos])
 
         string = self._parse_string()
@@ -6914,7 +6930,7 @@ def __init__(self, asCode: bool) -> None:
         if asCode:
             # render the expression as inline code
             self.class_type = 'cpp-expr'
-            self.node_type = nodes.literal  # type: Type[TextElement]
+            self.node_type = cast(Type[TextElement], nodes.literal)
         else:
             # render the expression as inline text
             self.class_type = 'cpp-texpr'
diff --git a/tests/roots/test-domain-cpp/conf.py b/tests/roots/test-domain-cpp/conf.py
index a45d22e2821..59b81816d40 100644
--- a/tests/roots/test-domain-cpp/conf.py
+++ b/tests/roots/test-domain-cpp/conf.py
@@ -1 +1,2 @@
 exclude_patterns = ['_build']
+cpp_id_attributes = ['inline']
diff --git a/tests/roots/test-domain-cpp/udl-negative.rst b/tests/roots/test-domain-cpp/udl-negative.rst
new file mode 100644
index 00000000000..7351d6a4f76
--- /dev/null
+++ b/tests/roots/test-domain-cpp/udl-negative.rst
@@ -0,0 +1,18 @@
+Invalid user-defined literal suffixes
+=====================================
+
+.. cpp:var:: constexpr auto bad_mixed = 1.0f_qs
+
+.. cpp:var:: constexpr auto bad_order = 1llu_qs
+
+.. cpp:var:: constexpr auto bad_digit_suffix = 1e-34q1
+
+.. cpp:var:: constexpr auto bad_symbol = 8_invalid*
+
+.. note::
+
+   Literals with digit-prefixed suffixes such as ``1e-34_1q`` are valid and
+   covered by the positive tests; omitting the underscore as in ``1e-34q1`` is
+   invalid.
+
+.. cpp:var:: constexpr auto bad_ws = 1 ud
diff --git a/tests/roots/test-domain-cpp/udl-numeric.rst b/tests/roots/test-domain-cpp/udl-numeric.rst
new file mode 100644
index 00000000000..99fdfacede1
--- /dev/null
+++ b/tests/roots/test-domain-cpp/udl-numeric.rst
@@ -0,0 +1,47 @@
+Numeric user-defined literals
+==============================
+
+.. cpp:var:: constexpr auto udl_decimal = 42_km
+
+.. cpp:var:: constexpr auto udl_hex = 0x2A_km
+
+.. cpp:var:: constexpr auto udl_binary = 0b1010_unit
+
+.. cpp:var:: constexpr auto udl_float = 3.14_q_s
+
+.. cpp:var:: constexpr auto hf1 = 0x1.0p+2ud
+
+.. cpp:var:: constexpr auto hf2 = 0x1p2_ud
+
+.. cpp:var:: constexpr auto iu1 = 42f_s
+
+.. cpp:var:: constexpr auto iu2 = 10Funit
+
+.. cpp:var:: constexpr auto udl_digit_float = 1e-34_1q
+
+.. cpp:var:: constexpr auto udl_digit_int = 42_1q
+
+.. cpp:namespace:: units::si
+
+.. cpp:var:: inline constexpr auto planck_constant = 6.62607015e-34q_J * 1q_s
+
+
+Builtin literal suffix regression
+---------------------------------
+
+.. cpp:var:: constexpr auto builtin_u = 1u
+
+.. cpp:var:: constexpr auto builtin_U = 1U
+
+.. cpp:var:: constexpr auto builtin_l = 1l
+
+.. cpp:var:: constexpr auto builtin_L = 1L
+
+.. cpp:var:: constexpr auto builtin_f = 1.0f
+
+.. cpp:var:: constexpr auto builtin_F = 1.0F
+
+.. note::
+
+   Digit separator examples such as ``1'000_km`` remain unsupported by the
+   test fixtures due to the base literal regular expressions.
diff --git a/tests/test_domain_cpp.py b/tests/test_domain_cpp.py
index 9db741ae5d9..3987f622831 100644
--- a/tests/test_domain_cpp.py
+++ b/tests/test_domain_cpp.py
@@ -921,6 +921,20 @@ def test_build_domain_cpp_semicolon(app, status, warning):
     assert len(ws) == 0
 
 
+@pytest.mark.sphinx(testroot='domain-cpp', confoverrides={'nitpicky': True}, freshenv=True)
+def test_build_domain_cpp_udl_numeric(app, status, warning):
+    app.builder.build_all()
+    ws = filter_warnings(warning, "udl-numeric")
+    assert len(ws) == 0
+
+
+@pytest.mark.sphinx(testroot='domain-cpp', confoverrides={'nitpicky': True}, freshenv=True)
+def test_build_domain_cpp_udl_negative(app, status, warning):
+    app.builder.build_all()
+    ws = filter_warnings(warning, "udl-negative")
+    assert len(ws) == 4
+
+
 @pytest.mark.sphinx(testroot='domain-cpp',
                     confoverrides={'nitpicky': True, 'strip_signature_backslash': True})
 def test_build_domain_cpp_backslash_ok(app, status, warning):