diff --git a/pyinaturalist/rest_api.py b/pyinaturalist/rest_api.py
index e9edc76..d1297a3 100644
--- a/pyinaturalist/rest_api.py
+++ b/pyinaturalist/rest_api.py
@@ -10,6 +10,7 @@ Functions
     :nosignatures:
 
 """
+import os
 from time import sleep
 from typing import Dict, Any, List, Union
 
@@ -39,7 +40,11 @@ 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.
@@ -51,12 +56,38 @@ def get_access_token(
         >>> headers = {"Authorization": f"Bearer {access_token}"}
 
     Args:
-        username: iNaturalist username
-        password: iNaturalist password
-        app_id: iNaturalist application ID
-        app_secret: iNaturalist application secret
+        username: iNaturalist username. Can also be provided via the
+            ``INAT_USERNAME`` environment variable.
+        password: iNaturalist password. Can also be provided via the
+            ``INAT_PASSWORD`` environment variable.
+        app_id: iNaturalist application ID. Can also be provided via the
+            ``INAT_APP_ID`` environment variable.
+        app_secret: iNaturalist application secret. Can also be provided via the
+            ``INAT_APP_SECRET`` environment variable.
         user_agent: a user-agent string that will be passed to iNaturalist.
     """
+    # Use provided arguments, falling back to environment variables
+    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")
+
+    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 required credentials: {', '.join(missing)}. "
+            "Provide them as arguments or via environment variables "
+            "(INAT_USERNAME, INAT_PASSWORD, INAT_APP_ID, INAT_APP_SECRET)."
+        )
+
     payload = {
         "client_id": app_id,
         "client_secret": app_secret,