diff --git a/sphinx/domains/c.py b/sphinx/domains/c.py
index 0869d69119a..c487c17d545 100644
--- a/sphinx/domains/c.py
+++ b/sphinx/domains/c.py
@@ -106,6 +106,7 @@
     # Floating-point
     # --------------
     |(float|double|long\s+double)(\s+(_Complex|complex|_Imaginary|imaginary))?
+    |(_Complex|complex|_Imaginary|imaginary)\s+(float|double|long\s+double)
     |_Decimal(32|64|128)
     # extensions
     |__float80|_Float64x|__float128|_Float128|__ibm128
diff --git a/sphinx/domains/cpp.py b/sphinx/domains/cpp.py
index 36455c69a30..0d8aee019f1 100644
--- a/sphinx/domains/cpp.py
+++ b/sphinx/domains/cpp.py
@@ -349,6 +349,7 @@
     # Floating-point
     # --------------
     |(float|double|long\s+double)(\s+(_Complex|_Imaginary))?
+    |(_Complex|_Imaginary)\s+(float|double|long\s+double)
     # extensions
     |__float80|_Float64x|__float128|_Float128
     # Integer types that could be prefixes of the previous ones
@@ -482,16 +483,14 @@
     'float': 'f',
     'double': 'd',
     'long double': 'e',
-    '__float80': 'e',
-    '_Float64x': 'e',
-    '__float128': 'g',
-    '_Float128': 'g',
-    'float _Complex': 'Cf',
-    'double _Complex': 'Cd',
-    'long double _Complex': 'Ce',
-    'float _Imaginary': 'f',
-    'double _Imaginary': 'd',
-    'long double _Imaginary': 'e',
+    '__float80': 'e', '_Float64x': 'e',
+    '__float128': 'g', '_Float128': 'g',
+    'float _Complex': 'Cf', '_Complex float': 'Cf',
+    'double _Complex': 'Cd', '_Complex double': 'Cd',
+    'long double _Complex': 'Ce', '_Complex long double': 'Ce',
+    'float _Imaginary': 'f', '_Imaginary float': 'f',
+    'double _Imaginary': 'd', '_Imaginary double': 'd',
+    'long double _Imaginary': 'e', '_Imaginary long double': 'e',
     'auto': 'Da',
     'decltype(auto)': 'Dc',
     'std::nullptr_t': 'Dn'
diff --git a/tests/test_domain_c.py b/tests/test_domain_c.py
index a9aa55687d2..0800c505962 100644
--- a/tests/test_domain_c.py
+++ b/tests/test_domain_c.py
@@ -306,7 +306,8 @@ def signed(t):
         for f in ('float', 'double', 'long double'):
             yield f
             yield from (f + "  _Complex", f + "  complex")
-            yield from (f + "  _Imaginary", f + "  imaginary")
+            yield from ("_Complex  " + f, "complex  " + f)
+            yield from ("_Imaginary  " + f, "imaginary  " + f)
         # extensions
         # https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html#Floating-Types
         yield from ('__float80', '_Float64x',