diff --git a/doc/source/whatsnew/v0.17.0.txt b/doc/source/whatsnew/v0.17.0.txt
index f96a2954f98a2..3d27e840232f7 100644
--- a/doc/source/whatsnew/v0.17.0.txt
+++ b/doc/source/whatsnew/v0.17.0.txt
@@ -95,3 +95,7 @@ Bug Fixes
 
 - Bug where infer_freq infers timerule (WOM-5XXX) unsupported by to_offset (:issue:`9425`)
 
+
+- Bug in ``to_csv`` where ``data_format`` is ignored if the ``datetime`` is not 
+an integral date value (i.e. it is fractional) (:issue:`10209`)
+
diff --git a/pandas/core/format.py b/pandas/core/format.py
index 3ab41ded1deea..429051b5815fa 100644
--- a/pandas/core/format.py
+++ b/pandas/core/format.py
@@ -2114,7 +2114,7 @@ def _get_format_datetime64_from_values(values, date_format):
     is_dates_only = _is_dates_only(values)
     if is_dates_only:
         return date_format or "%Y-%m-%d"
-    return None
+    return date_format
 
 
 class Timedelta64Formatter(GenericArrayFormatter):
diff --git a/pandas/tests/test_format.py b/pandas/tests/test_format.py
index a7129bca59a7f..4d21190e7a50d 100644
--- a/pandas/tests/test_format.py
+++ b/pandas/tests/test_format.py
@@ -2449,6 +2449,27 @@ def test_to_csv_decimal(self):
         expected_float_format = ';col1;col2;col3\n0;1;a;10,10\n'
         self.assertEqual(df.to_csv(decimal=',',sep=';', float_format = '%.2f'), expected_float_format)
 
+    def test_to_csv_date_format(self):
+        # GH 10209
+        df_sec = DataFrame({'A': pd.date_range('20130101',periods=5,freq='s')})
+        df_day = DataFrame({'A': pd.date_range('20130101',periods=5,freq='d')})
+
+        expected_default_sec = ',A\n0,2013-01-01 00:00:00\n1,2013-01-01 00:00:01\n2,2013-01-01 00:00:02' + \
+                               '\n3,2013-01-01 00:00:03\n4,2013-01-01 00:00:04\n'
+        self.assertEqual(df_sec.to_csv(), expected_default_sec)
+
+        expected_ymdhms_day = ',A\n0,2013-01-01 00:00:00\n1,2013-01-02 00:00:00\n2,2013-01-03 00:00:00' + \
+                              '\n3,2013-01-04 00:00:00\n4,2013-01-05 00:00:00\n'
+        self.assertEqual(df_day.to_csv(date_format='%Y-%m-%d %H:%M:%S'), expected_ymdhms_day)
+
+        expected_ymd_sec = ',A\n0,2013-01-01\n1,2013-01-01\n2,2013-01-01\n3,2013-01-01\n4,2013-01-01\n'
+        self.assertEqual(df_sec.to_csv(date_format='%Y-%m-%d'), expected_ymd_sec)
+
+        expected_default_day = ',A\n0,2013-01-01\n1,2013-01-02\n2,2013-01-03\n3,2013-01-04\n4,2013-01-05\n'
+        self.assertEqual(df_day.to_csv(), expected_default_day)
+        self.assertEqual(df_day.to_csv(date_format='%Y-%m-%d'), expected_default_day)
+
+
 class TestSeriesFormatting(tm.TestCase):
     _multiprocess_can_split_ = True