diff --git a/.github/workflows/ci_linux.yml b/.github/workflows/ci_linux.yml
index fd7ef29be..305c30e2d 100644
--- a/.github/workflows/ci_linux.yml
+++ b/.github/workflows/ci_linux.yml
@@ -87,7 +87,7 @@ jobs:
 
           # Additional env variables
           echo "GDAL_DATA=$GDALINST/gdal-$GDALVERSION/share/gdal" >> $GITHUB_ENV
-          echo "PROJ_LIB=$GDALINST/gdal-$GDALVERSION/share/proj" >> $GITHUB_ENV
+          echo "PROJ_DATA=$GDALINST/gdal-$GDALVERSION/share/proj" >> $GITHUB_ENV
           echo "LD_LIBRARY_PATH=$GDALINST/gdal-$GDALVERSION/lib:$GDALINST/proj-$PROJVERSION/lib:$FILEGDB/lib:\$LD_LIBRARY_PATH" >> $GITHUB_ENV
 
           # Add PATH
diff --git a/README.rst b/README.rst
index f9b2160bb..3c3975dec 100644
--- a/README.rst
+++ b/README.rst
@@ -304,7 +304,7 @@ Note: The following environment variables needs to be set so that Fiona works co
   Windows ``PATH`` (e.g. ``C:\gdal\bin``).
 * The gdal-data directory needs to be in your Windows ``PATH`` or the environment variable
   ``GDAL_DATA`` must be set (e.g. ``C:\gdal\bin\gdal-data``).
-* The environment variable ``PROJ_LIB`` must be set to the proj library directory (e.g.
+* The environment variable ``PROJ_LIB`` (PROJ < 9.1) | ``PROJ_DATA`` (PROJ 9.1+) must be set to the proj data directory (e.g.
   ``C:\gdal\bin\proj6\share``)
 
 The `Appveyor CI build <https://ci.appveyor.com/project/sgillies/fiona/history>`__
diff --git a/fiona/_env.pyx b/fiona/_env.pyx
index f388ddd26..49f86509f 100644
--- a/fiona/_env.pyx
+++ b/fiona/_env.pyx
@@ -430,7 +430,13 @@ cdef class GDALEnv(ConfigEnv):
                                 log.debug("GDAL data found in other locations: path=%r.", path)
                                 self.update_config_options(GDAL_DATA=path)
 
-                    if 'PROJ_LIB' in os.environ:
+                    if 'PROJ_DATA' in os.environ:
+                        # PROJ 9.1+
+                        log.debug("PROJ_DATA found in environment.")
+                        path = os.environ["PROJ_DATA"]
+                        set_proj_data_search_path(path)
+                    elif 'PROJ_LIB' in os.environ:
+                        # PROJ < 9.1
                         log.debug("PROJ_LIB found in environment.")
                         path = os.environ["PROJ_LIB"]
                         set_proj_data_search_path(path)
diff --git a/fiona/env.py b/fiona/env.py
index fe9b97903..e531d3d72 100644
--- a/fiona/env.py
+++ b/fiona/env.py
@@ -683,7 +683,13 @@ def wrapper(*args, **kwds):
             set_gdal_config("GDAL_DATA", path)
             log.debug("GDAL data found in other locations: path=%r.", path)
 
-if "PROJ_LIB" in os.environ:
+if 'PROJ_DATA' in os.environ:
+    # PROJ 9.1+
+    path = os.environ["PROJ_DATA"]
+    set_proj_data_search_path(path)
+
+elif "PROJ_LIB" in os.environ:
+    # PROJ < 9.1
     path = os.environ["PROJ_LIB"]
     set_proj_data_search_path(path)
 
diff --git a/fiona/fio/env.py b/fiona/fio/env.py
index a8de1b422..b7f71e93d 100644
--- a/fiona/fio/env.py
+++ b/fiona/fio/env.py
@@ -37,4 +37,4 @@ def env(ctx, key):
         elif key == 'gdal_data':
             click.echo(os.environ.get('GDAL_DATA') or GDALDataFinder().search())
         elif key == 'proj_data':
-            click.echo(os.environ.get('PROJ_LIB') or PROJDataFinder().search())
+            click.echo(os.environ.get('PROJ_DATA', os.environ.get('PROJ_LIB')) or PROJDataFinder().search())
diff --git a/setup.py b/setup.py
index a0054c3d5..b5c71fbf8 100644
--- a/setup.py
+++ b/setup.py
@@ -132,8 +132,8 @@ def copy_data_tree(datadir, destdir):
                 log.info("Copying gdal data from %s" % gdal_data)
                 copy_data_tree(gdal_data, destdir)
 
-        # Conditionally copy PROJ.4 data.
-        projdatadir = os.environ.get('PROJ_LIB', '/usr/local/share/proj')
+        # Conditionally copy PROJ DATA.
+        projdatadir = os.environ.get('PROJ_DATA', os.environ.get('PROJ_LIB', '/usr/local/share/proj'))
         if os.path.exists(projdatadir):
             log.info("Copying proj data from %s" % projdatadir)
             copy_data_tree(projdatadir, 'fiona/proj_data')
diff --git a/tests/test_data_paths.py b/tests/test_data_paths.py
index 398d65b8c..f86e23949 100644
--- a/tests/test_data_paths.py
+++ b/tests/test_data_paths.py
@@ -46,8 +46,11 @@ def test_env_gdal_data_environ(monkeypatch):
     assert result.output.strip() == '/foo/bar'
 
 
-def test_env_proj_data_environ(monkeypatch):
-    monkeypatch.setenv('PROJ_LIB', '/foo/bar')
+@pytest.mark.parametrize("data_directory_env", ["PROJ_LIB", "PROJ_DATA"])
+def test_env_proj_data_environ(data_directory_env, monkeypatch):
+    monkeypatch.delenv('PROJ_DATA', raising=False)
+    monkeypatch.delenv('PROJ_LIB', raising=False)
+    monkeypatch.setenv(data_directory_env, '/foo/bar')
     runner = CliRunner()
     result = runner.invoke(main_group, ['env', '--proj-data'])
     assert result.exit_code == 0