diff --git a/doc/source/whatsnew/v0.25.0.rst b/doc/source/whatsnew/v0.25.0.rst
index ddc5e543c6165..8dbfa2a898694 100644
--- a/doc/source/whatsnew/v0.25.0.rst
+++ b/doc/source/whatsnew/v0.25.0.rst
@@ -136,7 +136,7 @@ Categorical
 Datetimelike
 ^^^^^^^^^^^^
 
--
+- Bug with :class:`Timestamp` and :class:`CustomBusinessDay` arithmetic throwing an exception with datetime subclasses (:issue:`25734`)
 -
 -
 
diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index 1c0adaaa288a9..5c1ea95676897 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -380,9 +380,12 @@ cdef _TSObject convert_datetime_to_tsobject(datetime ts, object tz,
         obj.value -= int(offset.total_seconds() * 1e9)
 
     if not PyDateTime_CheckExact(ts):
-        # datetime instance but not datetime type --> Timestamp
-        obj.value += ts.nanosecond
-        obj.dts.ps = ts.nanosecond * 1000
+        try:
+            obj.value += ts.nanosecond
+            obj.dts.ps = ts.nanosecond * 1000
+        except AttributeError:
+            # probably a subclass of datetime
+            pass
 
     if nanos:
         obj.value += nanos
@@ -608,8 +611,11 @@ cpdef inline datetime localize_pydatetime(datetime dt, object tz):
     if tz is None:
         return dt
     elif not PyDateTime_CheckExact(dt):
-        # i.e. is a Timestamp
-        return dt.tz_localize(tz)
+        try:
+            return dt.tz_localize(tz)
+        except AttributeError:
+            # probably a subclass of datetime
+            pass
     elif is_utc(tz):
         return _localize_pydatetime(dt, tz)
     try:
diff --git a/pandas/tests/arithmetic/test_datetime64.py b/pandas/tests/arithmetic/test_datetime64.py
index c81a371f37dc1..4ee635025fbe1 100644
--- a/pandas/tests/arithmetic/test_datetime64.py
+++ b/pandas/tests/arithmetic/test_datetime64.py
@@ -5,6 +5,7 @@
 from datetime import datetime, timedelta
 from itertools import product, starmap
 import operator
+import sys
 import warnings
 
 import numpy as np
@@ -23,6 +24,8 @@
 from pandas.core.indexes.datetimes import _to_M8
 import pandas.util.testing as tm
 
+from pandas.tseries.offsets import CustomBusinessDay
+
 
 def assert_all(obj):
     """
@@ -2350,3 +2353,38 @@ def test_shift_months(years, months):
            for x in dti]
     expected = DatetimeIndex(raw)
     tm.assert_index_equal(actual, expected)
+
+
+def test_add_with_monkeypatched_datetime(monkeypatch):
+    # GH 25734
+
+    class MetaDatetime(type):
+        @classmethod
+        def __instancecheck__(self, obj):
+            return isinstance(obj, datetime)
+
+    class FakeDatetime(MetaDatetime("NewBase", (datetime,), {})):
+        pass
+
+    with monkeypatch.context() as m:
+        # monkeypatch datetime everywhere
+        for mod_name, module in list(sys.modules.items()):
+            try:
+                if (mod_name == __name__ or
+                   module.__name__ in ('datetime',)):
+                    continue
+                module_attributes = dir(module)
+            except (ImportError, AttributeError, TypeError):
+                continue
+            for attribute_name in module_attributes:
+                try:
+                    attribute_value = getattr(module, attribute_name)
+                except (ImportError, AttributeError, TypeError):
+                    continue
+                if id(datetime) == id(attribute_value):
+                    m.setattr(module, attribute_name, FakeDatetime)
+
+        dt = FakeDatetime(2000, 1, 1, tzinfo=pytz.UTC)
+        result = Timestamp(dt) + CustomBusinessDay()
+        expected = Timestamp("2000-01-03", tzinfo=pytz.UTC)
+        assert result == expected
diff --git a/pandas/tests/tslibs/test_conversion.py b/pandas/tests/tslibs/test_conversion.py
index 13398a69b4982..4b4345da18d1c 100644
--- a/pandas/tests/tslibs/test_conversion.py
+++ b/pandas/tests/tslibs/test_conversion.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 
+from datetime import datetime
+
 import numpy as np
 import pytest
 from pytz import UTC
@@ -7,7 +9,7 @@
 from pandas._libs.tslib import iNaT
 from pandas._libs.tslibs import conversion, timezones
 
-from pandas import date_range
+from pandas import Timestamp, date_range
 import pandas.util.testing as tm
 
 
@@ -66,3 +68,22 @@ def test_length_zero_copy(dtype, copy):
     arr = np.array([], dtype=dtype)
     result = conversion.ensure_datetime64ns(arr, copy=copy)
     assert result.base is (None if copy else arr)
+
+
+class FakeDatetime(datetime):
+    pass
+
+
+@pytest.mark.parametrize("dt, expected", [
+    pytest.param(Timestamp("2000-01-01"),
+                 Timestamp("2000-01-01", tz=UTC), id="timestamp"),
+    pytest.param(datetime(2000, 1, 1),
+                 datetime(2000, 1, 1, tzinfo=UTC),
+                 id="datetime"),
+    pytest.param(FakeDatetime(2000, 1, 1),
+                 FakeDatetime(2000, 1, 1, tzinfo=UTC),
+                 id="fakedatetime")])
+def test_localize_pydatetime_dt_types(dt, expected):
+    # GH 25734
+    result = conversion.localize_pydatetime(dt, UTC)
+    assert result == expected