diff --git a/pandas/core/strings.py b/pandas/core/strings.py
index c824ad1712a5a..18a83269a2f0f 100644
--- a/pandas/core/strings.py
+++ b/pandas/core/strings.py
@@ -2074,9 +2074,10 @@ def cat(self, others=None, sep=None, na_rep=None, join=None):
 
             If others is None, the method returns the concatenation of all
             strings in the calling Series/Index.
-        sep : string or None, default None
-            If None, concatenates without any separator.
-        na_rep : string or None, default None
+        sep : str, default ''
+            The separator between the different elements/columns. By default
+            the empty string `''` is used.
+        na_rep : str or None, default None
             Representation that is inserted for all missing values:
 
             - If `na_rep` is None, and `others` is None, missing values in the
diff --git a/pandas/tests/test_strings.py b/pandas/tests/test_strings.py
index 87bf89229a64e..f0873eb7683e9 100644
--- a/pandas/tests/test_strings.py
+++ b/pandas/tests/test_strings.py
@@ -162,10 +162,11 @@ def test_str_cat_raises_intuitive_error(self, box):
         with tm.assert_raises_regex(ValueError, message):
             s.str.cat('    ')
 
+    @pytest.mark.parametrize('sep', ['', None])
     @pytest.mark.parametrize('dtype_target', ['object', 'category'])
     @pytest.mark.parametrize('dtype_caller', ['object', 'category'])
     @pytest.mark.parametrize('box', [Series, Index])
-    def test_str_cat_categorical(self, box, dtype_caller, dtype_target):
+    def test_str_cat_categorical(self, box, dtype_caller, dtype_target, sep):
         s = Index(['a', 'a', 'b', 'a'], dtype=dtype_caller)
         s = s if box == Index else Series(s, index=s)
         t = Index(['b', 'a', 'b', 'c'], dtype=dtype_target)
@@ -176,23 +177,23 @@ def test_str_cat_categorical(self, box, dtype_caller, dtype_target):
         # Series/Index with unaligned Index
         with tm.assert_produces_warning(expected_warning=FutureWarning):
             # FutureWarning to switch to alignment by default
-            result = s.str.cat(t)
+            result = s.str.cat(t, sep=sep)
             assert_series_or_index_equal(result, expected)
 
         # Series/Index with Series having matching Index
         t = Series(t, index=s)
-        result = s.str.cat(t)
+        result = s.str.cat(t, sep=sep)
         assert_series_or_index_equal(result, expected)
 
         # Series/Index with Series.values
-        result = s.str.cat(t.values)
+        result = s.str.cat(t.values, sep=sep)
         assert_series_or_index_equal(result, expected)
 
         # Series/Index with Series having different Index
         t = Series(t.values, index=t)
         with tm.assert_produces_warning(expected_warning=FutureWarning):
             # FutureWarning to switch to alignment by default
-            result = s.str.cat(t)
+            result = s.str.cat(t, sep=sep)
             assert_series_or_index_equal(result, expected)
 
     @pytest.mark.parametrize('box', [Series, Index])