diff --git a/doc/source/whatsnew/v1.5.0.rst b/doc/source/whatsnew/v1.5.0.rst
index ea5258cf1537d..ac1565d1e11c3 100644
--- a/doc/source/whatsnew/v1.5.0.rst
+++ b/doc/source/whatsnew/v1.5.0.rst
@@ -304,7 +304,7 @@ Conversion
 
 Strings
 ^^^^^^^
--
+- Bug in :meth:`str.startswith` and :meth:`str.endswith` when using other series as parameter _pat_. Now raises ``TypeError`` (:issue:`3485`)
 -
 
 Interval
diff --git a/pandas/core/strings/accessor.py b/pandas/core/strings/accessor.py
index 6a0fa3c311288..b7ccc087ff1f1 100644
--- a/pandas/core/strings/accessor.py
+++ b/pandas/core/strings/accessor.py
@@ -2282,6 +2282,9 @@ def startswith(self, pat, na=None):
         3    False
         dtype: bool
         """
+        if not isinstance(pat, str):
+            msg = f"expected a string object, not {type(pat).__name__}"
+            raise TypeError(msg)
         result = self._data.array._str_startswith(pat, na=na)
         return self._wrap_result(result, returns_string=False)
 
@@ -2339,6 +2342,9 @@ def endswith(self, pat, na=None):
         3    False
         dtype: bool
         """
+        if not isinstance(pat, str):
+            msg = f"expected a string object, not {type(pat).__name__}"
+            raise TypeError(msg)
         result = self._data.array._str_endswith(pat, na=na)
         return self._wrap_result(result, returns_string=False)
 
diff --git a/pandas/tests/strings/test_strings.py b/pandas/tests/strings/test_strings.py
index 9a82110f65f83..b061ef0bd8a15 100644
--- a/pandas/tests/strings/test_strings.py
+++ b/pandas/tests/strings/test_strings.py
@@ -16,6 +16,17 @@
 import pandas._testing as tm
 
 
+@pytest.mark.parametrize("pattern", [0, True, Series(["foo", "bar"])])
+def test_startswith_endswith_non_str_patterns(pattern):
+    # GH3485
+    ser = Series(["foo", "bar"])
+    msg = f"expected a string object, not {type(pattern).__name__}"
+    with pytest.raises(TypeError, match=msg):
+        ser.str.startswith(pattern)
+    with pytest.raises(TypeError, match=msg):
+        ser.str.endswith(pattern)
+
+
 def assert_series_or_index_equal(left, right):
     if isinstance(left, Series):
         tm.assert_series_equal(left, right)