diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 5c61f259a4202..d248131bd1c76 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -406,7 +406,7 @@ Conversion
 - Bug in :meth:`Series.view` and :meth:`Index.view` when converting between datetime-like (``datetime64[ns]``, ``datetime64[ns, tz]``, ``timedelta64``, ``period``) dtypes (:issue:`39788`)
 - Bug in creating a :class:`DataFrame` from an empty ``np.recarray`` not retaining the original dtypes (:issue:`40121`)
 - Bug in :class:`DataFrame` failing to raise ``TypeError`` when constructing from a ``frozenset`` (:issue:`40163`)
--
+- Bug in :meth:`~DataFrame.update` unnecessarily changing the dtype of a column (:issue:`4094`)
 
 Strings
 ^^^^^^^
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index f0f8d813bba96..897bfb2e1c588 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -6939,6 +6939,7 @@ def update(
         if not isinstance(other, DataFrame):
             other = DataFrame(other)
 
+        other_dtypes = other.dtypes
         other = other.reindex_like(self)
 
         for col in self.columns:
@@ -6963,7 +6964,11 @@ def update(
             if mask.all():
                 continue
 
-            self[col] = expressions.where(mask, this, that)
+            col_array = expressions.where(mask, this, that)
+            if self[col].dtype == other_dtypes[col] != col_array.dtype:
+                self[col] = Series(col_array, index=self.index, dtype=self[col].dtype)
+            else:
+                self[col] = col_array
 
     # ----------------------------------------------------------------------
     # Data reshaping
diff --git a/pandas/tests/frame/methods/test_update.py b/pandas/tests/frame/methods/test_update.py
index 408113e9bc417..6433691f7e884 100644
--- a/pandas/tests/frame/methods/test_update.py
+++ b/pandas/tests/frame/methods/test_update.py
@@ -146,3 +146,42 @@ def test_update_with_different_dtype(self):
 
         expected = DataFrame({"a": [1, 3], "b": [np.nan, 2], "c": ["foo", np.nan]})
         tm.assert_frame_equal(df, expected)
+
+    def test_update_with_subset_and_same_not_nullable_dtype(self, any_real_dtype):
+        # GH4094
+        df = DataFrame({"a": Series([1, 2, 3], dtype=any_real_dtype)})
+        update = df.copy()[:-1]
+        df.update(update)
+        assert df.a.dtype == any_real_dtype
+
+    def test_update_str_dtype(self, string_dtype):
+        # GH4094
+        df = DataFrame({"a": ["a", "b", "c"]}, dtype=string_dtype)
+        update = df.copy()
+        expected = df.copy()
+        df.update(update)
+        assert df.a.dtype == expected.a.dtype
+
+    def test_update_with_subset_str_dtype(self, string_dtype):
+        # GH4094
+        df = DataFrame({"a": ["a", "b", "c"]}, dtype=string_dtype)
+        update = df.copy()[:-1]
+        expected = df.copy()
+        df.update(update)
+        assert df.a.dtype == expected.a.dtype
+
+    def test_update_bool_dtype(self):
+        # GH4094
+        df = DataFrame({"a": [True, False, True]}, dtype=bool)
+        update = df.copy()
+        expected = df.copy()
+        df.update(update)
+        assert df.a.dtype == expected.a.dtype
+
+    def test_update_with_subset_bool_dtype(self):
+        # GH4094
+        df = DataFrame({"a": [True, False]}, dtype=bool)
+        update = DataFrame({"a": [False]}, dtype=bool)
+        expected = df.copy()
+        df.update(update)
+        assert df.a.dtype == expected.a.dtype