diff --git a/doc/source/whatsnew/v1.1.0.rst b/doc/source/whatsnew/v1.1.0.rst
index 84ad478226175..c14bcb6c6d8c5 100644
--- a/doc/source/whatsnew/v1.1.0.rst
+++ b/doc/source/whatsnew/v1.1.0.rst
@@ -796,13 +796,13 @@ ExtensionArray
 ^^^^^^^^^^^^^^
 
 - Fixed bug where :meth:`Series.value_counts` would raise on empty input of ``Int64`` dtype (:issue:`33317`)
+- Fixed bug where :meth:`StringArray.isna` would return ``False`` for NA values when ``pandas.options.mode.use_inf_as_na`` was set to ``True`` (:issue:`33655`)
 - Fixed bug in :class:`Series` construction with EA dtype and index but no data or scalar data fails (:issue:`26469`)
 - Fixed bug that caused :meth:`Series.__repr__()` to crash for extension types whose elements are multidimensional arrays (:issue:`33770`).
 - Fixed bug where :meth:`Series.update` would raise a ``ValueError`` for ``ExtensionArray`` dtypes with missing values (:issue:`33980`)
 - Fixed bug where :meth:`StringArray.memory_usage` was not implemented (:issue:`33963`)
 - Fixed bug that `DataFrame(columns=.., dtype='string')` would fail (:issue:`27953`, :issue:`33623`)
 
-
 Other
 ^^^^^
 - Appending a dictionary to a :class:`DataFrame` without passing ``ignore_index=True`` will raise ``TypeError: Can only append a dict if ignore_index=True``
diff --git a/pandas/_libs/missing.pyx b/pandas/_libs/missing.pyx
index 62a8bfccaa432..6bca5e370ac89 100644
--- a/pandas/_libs/missing.pyx
+++ b/pandas/_libs/missing.pyx
@@ -90,11 +90,6 @@ cpdef bint checknull_old(object val):
     return False
 
 
-cdef inline bint _check_none_nan_inf_neginf(object val):
-    return val is None or (isinstance(val, float) and
-                           (val != val or val == INF or val == NEGINF))
-
-
 @cython.wraparound(False)
 @cython.boundscheck(False)
 cpdef ndarray[uint8_t] isnaobj(ndarray arr):
@@ -141,6 +136,7 @@ def isnaobj_old(arr: ndarray) -> ndarray:
      - INF
      - NEGINF
      - NaT
+     - NA
 
     Parameters
     ----------
@@ -161,7 +157,7 @@ def isnaobj_old(arr: ndarray) -> ndarray:
     result = np.zeros(n, dtype=np.uint8)
     for i in range(n):
         val = arr[i]
-        result[i] = val is NaT or _check_none_nan_inf_neginf(val)
+        result[i] = checknull(val) or val == INF or val == NEGINF
     return result.view(np.bool_)
 
 
diff --git a/pandas/core/dtypes/missing.py b/pandas/core/dtypes/missing.py
index 048fd7adf55b1..ab8df492f1c01 100644
--- a/pandas/core/dtypes/missing.py
+++ b/pandas/core/dtypes/missing.py
@@ -17,6 +17,7 @@
     TD64NS_DTYPE,
     ensure_object,
     is_bool_dtype,
+    is_categorical_dtype,
     is_complex_dtype,
     is_datetimelike_v_numeric,
     is_dtype_equal,
@@ -209,8 +210,8 @@ def _isna_ndarraylike(obj, inf_as_na: bool = False):
     dtype = values.dtype
 
     if is_extension_array_dtype(dtype):
-        if inf_as_na:
-            result = values.isna() | (values == -np.inf) | (values == np.inf)
+        if inf_as_na and is_categorical_dtype(dtype):
+            result = libmissing.isnaobj_old(values.to_numpy())
         else:
             result = values.isna()
     elif is_string_dtype(dtype):
diff --git a/pandas/tests/arrays/string_/test_string.py b/pandas/tests/arrays/string_/test_string.py
index b681abf03a2b3..6f9a1a5be4c43 100644
--- a/pandas/tests/arrays/string_/test_string.py
+++ b/pandas/tests/arrays/string_/test_string.py
@@ -305,6 +305,28 @@ def test_value_counts_na():
     tm.assert_series_equal(result, expected)
 
 
+@pytest.mark.parametrize(
+    "values, expected",
+    [
+        (pd.array(["a", "b", "c"]), np.array([False, False, False])),
+        (pd.array(["a", "b", None]), np.array([False, False, True])),
+    ],
+)
+def test_use_inf_as_na(values, expected):
+    # https://github.com/pandas-dev/pandas/issues/33655
+    with pd.option_context("mode.use_inf_as_na", True):
+        result = values.isna()
+        tm.assert_numpy_array_equal(result, expected)
+
+        result = pd.Series(values).isna()
+        expected = pd.Series(expected)
+        tm.assert_series_equal(result, expected)
+
+        result = pd.DataFrame(values).isna()
+        expected = pd.DataFrame(expected)
+        tm.assert_frame_equal(result, expected)
+
+
 def test_memory_usage():
     # GH 33963
     series = pd.Series(["a", "b", "c"], dtype="string")
diff --git a/pandas/tests/extension/base/missing.py b/pandas/tests/extension/base/missing.py
index 2393d2edcd2c6..a5969ef961bab 100644
--- a/pandas/tests/extension/base/missing.py
+++ b/pandas/tests/extension/base/missing.py
@@ -127,3 +127,10 @@ def test_fillna_fill_other(self, data):
         expected = pd.DataFrame({"A": data, "B": [0.0] * len(result)})
 
         self.assert_frame_equal(result, expected)
+
+    def test_use_inf_as_na_no_effect(self, data_missing):
+        ser = pd.Series(data_missing)
+        expected = ser.isna()
+        with pd.option_context("mode.use_inf_as_na", True):
+            result = ser.isna()
+        self.assert_series_equal(result, expected)
diff --git a/pandas/tests/series/test_missing.py b/pandas/tests/series/test_missing.py
index 3b8c2e595148e..162778e372426 100644
--- a/pandas/tests/series/test_missing.py
+++ b/pandas/tests/series/test_missing.py
@@ -509,12 +509,12 @@ def test_fillna_nat(self):
         tm.assert_frame_equal(filled2, expected)
 
     def test_isna_for_inf(self):
-        s = Series(["a", np.inf, np.nan, 1.0])
+        s = Series(["a", np.inf, np.nan, pd.NA, 1.0])
         with pd.option_context("mode.use_inf_as_na", True):
             r = s.isna()
             dr = s.dropna()
-        e = Series([False, True, True, False])
-        de = Series(["a", 1.0], index=[0, 3])
+        e = Series([False, True, True, True, False])
+        de = Series(["a", 1.0], index=[0, 4])
         tm.assert_series_equal(r, e)
         tm.assert_series_equal(dr, de)