diff --git a/pyinaturalist/rest_api.py b/pyinaturalist/rest_api.py
index e9edc76..53b1b55 100644
--- a/pyinaturalist/rest_api.py
+++ b/pyinaturalist/rest_api.py
@@ -39,10 +39,17 @@ from pyinaturalist.response_format import convert_lat_long_to_float
 
 
 def get_access_token(
-    username: str, password: str, app_id: str, app_secret: str, user_agent: str = None
+    username: str = None,
+    password: str = None,
+    app_id: str = None,
+    app_secret: str = None,
+    user_agent: str = None,
 ) -> str:
     """Get an access token using the user's iNaturalist username and password.
     You still need an iNaturalist app to do this.
+    Credentials can be provided as arguments or via environment variables:
+    INAT_USERNAME, INAT_PASSWORD, INAT_APP_ID, INAT_APP_SECRET.
+    Arguments take precedence over environment variables.
 
     **API reference:** https://www.inaturalist.org/pages/api+reference#auth
 
@@ -57,6 +64,31 @@ def get_access_token(
         app_secret: iNaturalist application secret
         user_agent: a user-agent string that will be passed to iNaturalist.
     """
+    import os
+    
+    # Get values from arguments or environment variables, with args taking precedence
+    username = username or os.environ.get('INAT_USERNAME')
+    password = password or os.environ.get('INAT_PASSWORD')
+    app_id = app_id or os.environ.get('INAT_APP_ID')
+    app_secret = app_secret or os.environ.get('INAT_APP_SECRET')
+    
+    # Check that all required credentials are present
+    if not all([username, password, app_id, app_secret]):
+        missing = []
+        if not username:
+            missing.append('username')
+        if not password:
+            missing.append('password')
+        if not app_id:
+            missing.append('app_id')
+        if not app_secret:
+            missing.append('app_secret')
+        raise AuthenticationError(
+            f"Missing credentials: {', '.join(missing)}. "
+            "Provide them as arguments or set the INAT_USERNAME, INAT_PASSWORD, "
+            "INAT_APP_ID, and INAT_APP_SECRET environment variables."
+        )
+
     payload = {
         "client_id": app_id,
         "client_secret": app_secret,