diff --git a/geopandas/geodataframe.py b/geopandas/geodataframe.py
index fc97149..b7cbda7 100644
--- a/geopandas/geodataframe.py
+++ b/geopandas/geodataframe.py
@@ -1043,9 +1043,7 @@ box': (2.0, 1.0, 2.0, 1.0)}], 'bbox': (1.0, 1.0, 2.0, 2.0)}
 
         _to_feather(self, path, index=index, compression=compression, **kwargs)
 
-    def to_file(
-        self, filename, driver="ESRI Shapefile", schema=None, index=None, **kwargs
-    ):
+    def to_file(self, filename, driver=None, schema=None, index=None, **kwargs):
         """Write the ``GeoDataFrame`` to a file.
 
         By default, an ESRI shapefile is written, but any OGR data source
@@ -1059,8 +1057,10 @@ box': (2.0, 1.0, 2.0, 1.0)}], 'bbox': (1.0, 1.0, 2.0, 2.0)}
         ----------
         filename : string
             File path or file handle to write to.
-        driver : string, default: 'ESRI Shapefile'
+        driver : string, default None
             The OGR format driver used to write the vector file.
+            If not specified, it attempts to infer it from the file extension.
+            If no extension is specified, it saves ESRI Shapefile to a folder.
         schema : dict, default: None
             If specified, the schema dictionary is passed to Fiona to
             better control how the file is written.
diff --git a/geopandas/geoseries.py b/geopandas/geoseries.py
index 2c68348..15305ff 100644
--- a/geopandas/geoseries.py
+++ b/geopandas/geoseries.py
@@ -476,7 +476,7 @@ class GeoSeries(GeoPandasBase, Series):
 
         return GeoDataFrame({"geometry": self}).__geo_interface__
 
-    def to_file(self, filename, driver="ESRI Shapefile", index=None, **kwargs):
+    def to_file(self, filename, driver=None, index=None, **kwargs):
         """Write the ``GeoSeries`` to a file.
 
         By default, an ESRI shapefile is written, but any OGR data source
@@ -486,8 +486,10 @@ class GeoSeries(GeoPandasBase, Series):
         ----------
         filename : string
             File path or file handle to write to.
-        driver : string, default: 'ESRI Shapefile'
+        driver : string, default None
             The OGR format driver used to write the vector file.
+            If not specified, it attempts to infer it from the file extension.
+            If no extension is specified, it saves ESRI Shapefile to a folder.
         index : bool, default None
             If True, write index into one or more columns (for MultiIndex).
             Default None writes the index into one or more columns only if
diff --git a/geopandas/io/file.py b/geopandas/io/file.py
index fbd17f8..acae8b3 100644
--- a/geopandas/io/file.py
+++ b/geopandas/io/file.py
@@ -1,4 +1,5 @@
 from distutils.version import LooseVersion
+from pathlib import Path
 
 import warnings
 import numpy as np
@@ -36,6 +37,28 @@ from urllib.parse import uses_netloc, uses_params, uses_relative
 _VALID_URLS = set(uses_relative + uses_netloc + uses_params)
 _VALID_URLS.discard("")
 
+_EXTENSION_TO_DRIVER = {
+    ".bna": "BNA",
+    ".dxf": "DXF",
+    ".csv": "CSV",
+    ".shp": "ESRI Shapefile",
+    ".dbf": "ESRI Shapefile",
+    ".json": "GeoJSON",
+    ".geojson": "GeoJSON",
+    ".geojsonl": "GeoJSONSeq",
+    ".geojsons": "GeoJSONSeq",
+    ".gpkg": "GPKG",
+    ".gml": "GML",
+    ".xml": "GML",
+    ".gpx": "GPX",
+    ".gtm": "GPSTrackMaker",
+    ".gtz": "GPSTrackMaker",
+    ".tab": "MapInfo File",
+    ".mif": "MapInfo File",
+    ".mid": "MapInfo File",
+    ".dgn": "DGN",
+}
+
 
 def _check_fiona(func):
     if fiona is None:
@@ -233,10 +256,28 @@ def to_file(*args, **kwargs):
     return _to_file(*args, **kwargs)
 
 
+def _detect_driver(path):
+    """
+    Attempt to auto-detect driver based on the extension
+    """
+    try:
+        # in case the path is a file handle
+        path = path.name
+    except AttributeError:
+        pass
+    try:
+        return _EXTENSION_TO_DRIVER[Path(path).suffix.lower()]
+    except KeyError:
+        # Assume it is a shapefile folder for now. In the future,
+        # will likely raise an exception when the expected
+        # folder writing behavior is more clearly defined.
+        return "ESRI Shapefile"
+
+
 def _to_file(
     df,
     filename,
-    driver="ESRI Shapefile",
+    driver=None,
     schema=None,
     index=None,
     mode="w",
@@ -255,8 +296,10 @@ def _to_file(
     df : GeoDataFrame to be written
     filename : string
         File path or file handle to write to.
-    driver : string, default 'ESRI Shapefile'
+    driver : string, default None
         The OGR format driver used to write the vector file.
+        If not specified, it attempts to infer it from the file extension.
+        If no extension is specified, it saves ESRI Shapefile to a folder.
     schema : dict, default None
         If specified, the schema dictionary is passed to Fiona to
         better control how the file is written. If None, GeoPandas
@@ -308,6 +351,9 @@ def _to_file(
     else:
         crs = df.crs
 
+    if driver is None:
+        driver = _detect_driver(filename)
+
     if driver == "ESRI Shapefile" and any([len(c) > 10 for c in df.columns.tolist()]):
         warnings.warn(
             "Column names longer than 10 characters will be truncated when saved to "

diff --git a/geopandas/io/tests/test_file.py b/geopandas/io/tests/test_file.py
index d49876a..5fed529 100644
--- a/geopandas/io/tests/test_file.py
+++ b/geopandas/io/tests/test_file.py
@@ -13,7 +13,7 @@ from shapely.geometry import Point, Polygon, box
 
 import geopandas
 from geopandas import GeoDataFrame, read_file
-from geopandas.io.file import fiona_env
+from geopandas.io.file import fiona_env, _detect_driver, _EXTENSION_TO_DRIVER
 
 from geopandas.testing import assert_geodataframe_equal, assert_geoseries_equal
 from geopandas.tests.util import PACKAGE_DIR, validate_boro_df
@@ -61,7 +61,22 @@ def df_points():
 # to_file tests
 # -----------------------------------------------------------------------------
 
-driver_ext_pairs = [("ESRI Shapefile", "shp"), ("GeoJSON", "geojson"), ("GPKG", "gpkg")]
+driver_ext_pairs = [
+    ("ESRI Shapefile", ".shp"),
+    ("GeoJSON", ".geojson"),
+    ("GPKG", ".gpkg"),
+    (None, ".shp"),
+    (None, ""),
+    (None, ".geojson"),
+    (None, ".gpkg"),
+]
+
+
+def assert_correct_driver(file_path, ext):
+    # check the expected driver
+    expected_driver = "ESRI Shapefile" if ext == "" else _EXTENSION_TO_DRIVER[ext]
+    with fiona.open(str(file_path)) as fds:
+        assert fds.driver == expected_driver
 
 
 @pytest.mark.parametrize("driver,ext", driver_ext_pairs)
@@ -76,13 +91,15 @@ def test_to_file(tmpdir, df_nybb, df_null, driver, ext):
     assert np.alltrue(df["BoroName"].values == df_nybb["BoroName"])
 
     # Write layer with null geometry out to file
-    tempfilename = os.path.join(str(tmpdir), "null_geom." + ext)
+    tempfilename = os.path.join(str(tmpdir), "null_geom" + ext)
     df_null.to_file(tempfilename, driver=driver)
     # Read layer back in
     df = GeoDataFrame.from_file(tempfilename)
     assert "geometry" in df
     assert len(df) == 2
     assert np.alltrue(df["Name"].values == df_null["Name"])
+    # check the expected driver
+    assert_correct_driver(tempfilename, ext)
 
 
 @pytest.mark.parametrize("driver,ext", driver_ext_pairs)
@@ -95,6 +112,8 @@ def test_to_file_pathlib(tmpdir, df_nybb, df_null, driver, ext):
     assert "geometry" in df
     assert len(df) == 5
     assert np.alltrue(df["BoroName"].values == df_nybb["BoroName"])
+    # check the expected driver
+    assert_correct_driver(temppath, ext)
 
 
 @pytest.mark.parametrize("driver,ext", driver_ext_pairs)
@@ -112,10 +131,12 @@ def test_to_file_bool(tmpdir, driver, ext):
 
     df.to_file(tempfilename, driver=driver)
     result = read_file(tempfilename)
-    if driver == "ESRI Shapefile":
+    if ext in (".shp", ""):
         # Shapefile does not support boolean, so is read back as int
         df["b"] = df["b"].astype("int64")
     assert_geodataframe_equal(result, df)
+    # check the expected driver
+    assert_correct_driver(tempfilename, ext)
 
 
 def test_to_file_datetime(tmpdir):
@@ -133,26 +154,30 @@ def test_to_file_datetime(tmpdir):
 def test_to_file_with_point_z(tmpdir, ext, driver):
     """Test that 3D geometries are retained in writes (GH #612)."""
 
-    tempfilename = os.path.join(str(tmpdir), "test_3Dpoint." + ext)
+    tempfilename = os.path.join(str(tmpdir), "test_3Dpoint" + ext)
     point3d = Point(0, 0, 500)
     point2d = Point(1, 1)
     df = GeoDataFrame({"a": [1, 2]}, geometry=[point3d, point2d], crs=_CRS)
     df.to_file(tempfilename, driver=driver)
     df_read = GeoDataFrame.from_file(tempfilename)
     assert_geoseries_equal(df.geometry, df_read.geometry)
+    # check the expected driver
+    assert_correct_driver(tempfilename, ext)
 
 
 @pytest.mark.parametrize("driver,ext", driver_ext_pairs)
 def test_to_file_with_poly_z(tmpdir, ext, driver):
     """Test that 3D geometries are retained in writes (GH #612)."""
 
-    tempfilename = os.path.join(str(tmpdir), "test_3Dpoly." + ext)
+    tempfilename = os.path.join(str(tmpdir), "test_3Dpoly" + ext)
     poly3d = Polygon([[0, 0, 5], [0, 1, 5], [1, 1, 5], [1, 0, 5]])
     poly2d = Polygon([[0, 0], [0, 1], [1, 1], [1, 0]])
     df = GeoDataFrame({"a": [1, 2]}, geometry=[poly3d, poly2d], crs=_CRS)
     df.to_file(tempfilename, driver=driver)
     df_read = GeoDataFrame.from_file(tempfilename)
     assert_geoseries_equal(df.geometry, df_read.geometry)
+    # check the expected driver
+    assert_correct_driver(tempfilename, ext)
 
 
 def test_to_file_types(tmpdir, df_points):
@@ -248,10 +273,11 @@ def test_append_file(tmpdir, df_nybb, df_null, driver, ext):
     """Test to_file with append mode and from_file"""
     from fiona import supported_drivers
 
+    tempfilename = os.path.join(str(tmpdir), "boros" + ext)
+    driver = driver if driver else _detect_driver(tempfilename)
     if "a" not in supported_drivers[driver]:
         return None
 
-    tempfilename = os.path.join(str(tmpdir), "boros." + ext)
     df_nybb.to_file(tempfilename, driver=driver)
     df_nybb.to_file(tempfilename, mode="a", driver=driver)
     # Read layer back in
@@ -262,7 +288,7 @@ def test_append_file(tmpdir, df_nybb, df_null, driver, ext):
     assert_geodataframe_equal(df, expected, check_less_precise=True)
 
     # Write layer with null geometry out to file
-    tempfilename = os.path.join(str(tmpdir), "null_geom." + ext)
+    tempfilename = os.path.join(str(tmpdir), "null_geom" + ext)
     df_null.to_file(tempfilename, driver=driver)
     df_null.to_file(tempfilename, mode="a", driver=driver)
     # Read layer back in
@@ -276,10 +302,10 @@ def test_append_file(tmpdir, df_nybb, df_null, driver, ext):
 @pytest.mark.parametrize("driver,ext", driver_ext_pairs)
 def test_empty_crs(tmpdir, driver, ext):
     """Test handling of undefined CRS with GPKG driver (GH #1975)."""
-    if driver == "GPKG":
+    if ext == ".gpkg":
         pytest.xfail("GPKG is read with Undefined geographic SRS.")
 
-    tempfilename = os.path.join(str(tmpdir), "boros." + ext)
+    tempfilename = os.path.join(str(tmpdir), "boros" + ext)
     df = GeoDataFrame(
         {
             "a": [1, 2, 3],
@@ -290,7 +316,7 @@ def test_empty_crs(tmpdir, driver, ext):
     df.to_file(tempfilename, driver=driver)
     result = read_file(tempfilename)
 
-    if driver == "GeoJSON":
+    if ext == ".geojson":
         # geojson by default assumes epsg:4326
         df.crs = "EPSG:4326"
 
@@ -802,3 +828,10 @@ def test_write_index_to_file(tmpdir, df_points, driver, ext):
     # named DatetimeIndex
     df.index.name = "datetime"
     do_checks(df, index_is_used=True)
+
+
+def test_to_file__undetermined_driver(tmp_path, df_nybb):
+    shpdir = tmp_path / "boros.invalid"
+    df_nybb.to_file(shpdir)
+    assert shpdir.is_dir()
+    assert list(shpdir.glob("*.shp"))

Use GPKG as the default driver for `to_file`?
Curios what the general thoughts are on switching to use geopackage as the default driver when writing to a file?

http://switchfromshapefile.org/#geopackage
geopandas/io/tests/test_file.py::test_read_file_remote_geojson_url geopandas/io/tests/test_file.py::test_read_file_filtered_with_gdf_boundary_mismatched_crs__mask geopandas/io/tests/test_file.py::test_read_file_textio geopandas/io/tests/test_file.py::test_read_file_filtered geopandas/io/tests/test_file.py::test_read_file_filtered_rows_invalid geopandas/io/tests/test_file.py::test_read_file_raw_stream geopandas/io/tests/test_file.py::test_read_file_filtered_with_gdf_boundary_mismatched_crs geopandas/io/tests/test_file.py::test_read_file__ignore_geometry geopandas/io/tests/test_file.py::test_read_file_filtered_with_gdf_boundary__mask geopandas/io/tests/test_file.py::test_read_file_empty_shapefile geopandas/io/tests/test_file.py::test_read_file_bytesio geopandas/io/tests/test_file.py::test_read_file__ignore_all_fields geopandas/io/tests/test_file.py::test_read_file_filtered__rows_bbox__polygon geopandas/io/tests/test_file.py::test_read_file_privacy geopandas/io/tests/test_file.py::test_read_file_tempfile geopandas/io/tests/test_file.py::test_read_file_filtered_with_gdf_boundary geopandas/io/tests/test_file.py::test_allow_legacy_gdal_path geopandas/io/tests/test_file.py::test_read_file_pathlib geopandas/io/tests/test_file.py::test_read_file geopandas/io/tests/test_file.py::test_read_file_filtered__rows geopandas/io/tests/test_file.py::test_read_file_filtered__rows_bbox geopandas/io/tests/test_file.py::test_infer_zipped_file geopandas/io/tests/test_file.py::test_read_file_filtered_with_gdf_boundary__mask__polygon