diff --git a/doc/source/whatsnew/v2.3.3.rst b/doc/source/whatsnew/v2.3.3.rst
index 323963a4ec4d2..48e08a11e517a 100644
--- a/doc/source/whatsnew/v2.3.3.rst
+++ b/doc/source/whatsnew/v2.3.3.rst
@@ -48,6 +48,7 @@ Bug fixes
 with a compiled regex and custom flags (:issue:`62240`)
 - Fix :meth:`Series.str.match` and :meth:`Series.str.fullmatch` not matching patterns with groups correctly for the Arrow-backed string dtype (:issue:`61072`)
 - Fix comparing a :class:`StringDtype` Series with mixed objects raising an error (:issue:`60228`)
+- Fix error being raised when using a numpy ufunc with a Python-backed string array (:issue:`40800`)
 
 Improvements and fixes for Copy-on-Write
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index ef64bda3dc504..fab51ffa56919 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -5,6 +5,7 @@
 Any,
 Literal,
 Self,
+ cast,
 )
 
 import numpy as np
@@ -48,6 +49,7 @@
 )
 
 from pandas import Index
+ from pandas.arrays import StringArray
 
 
 class NumpyExtensionArray(
@@ -234,6 +236,16 @@ def __array_ufunc__(self, ufunc: np.ufunc, method: str, *inputs, **kwargs):
 # e.g. test_np_max_nested_tuples
 return result
 else:
+ if self.dtype.type is str: # type: ignore[comparison-overlap]
+ # StringDtype
+ self = cast("StringArray", self)
+ try:
+ # specify dtype to preserve storage/na_value
+ return type(self)(result, dtype=self.dtype)
+ except ValueError:
+ # if validation of input fails (no strings)
+ # -> fallback to returning raw numpy array
+ return result
 # one return value; re-box array-like results
 return type(self)(result)
 
diff --git a/pandas/tests/arrays/string_/test_string.py b/pandas/tests/arrays/string_/test_string.py
index 1a71f6c41c4f1..d3effb7c33457 100644
--- a/pandas/tests/arrays/string_/test_string.py
+++ b/pandas/tests/arrays/string_/test_string.py
@@ -840,3 +840,30 @@ def test_string_array_view_type_error():
 arr = pd.array(["a", "b", "c"], dtype="string")
 with pytest.raises(TypeError, match="Cannot change data-type for string array."):
 arr.view("i8")
+
+
+@pytest.mark.parametrize("box", [pd.Series, pd.array])
+def test_numpy_array_ufunc(dtype, box):
+ arr = box(["a", "bb", "ccc"], dtype=dtype)
+
+ # custom ufunc that works with string (object) input -> returning numeric
+ str_len_ufunc = np.frompyfunc(lambda x: len(x), 1, 1)
+ result = str_len_ufunc(arr)
+ expected_cls = pd.Series if box is pd.Series else np.array
+ # TODO we should infer int64 dtype here?
+ expected = expected_cls([1, 2, 3], dtype=object)
+ tm.assert_equal(result, expected)
+
+ # custom ufunc returning strings
+ str_multiply_ufunc = np.frompyfunc(lambda x: x * 2, 1, 1)
+ result = str_multiply_ufunc(arr)
+ expected = box(["aa", "bbbb", "cccccc"], dtype=dtype)
+ if dtype.storage == "pyarrow":
+ # TODO ArrowStringArray should also preserve the class / dtype
+ if box is pd.array:
+ expected = np.array(["aa", "bbbb", "cccccc"], dtype=object)
+ else:
+ # not specifying the dtype because the exact dtype is not yet preserved
+ expected = pd.Series(["aa", "bbbb", "cccccc"])
+
+ tm.assert_equal(result, expected)
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