diff --git a/README.rst b/README.rst
index efb5a0b..1846064 100644
--- a/README.rst
+++ b/README.rst
@@ -52,6 +52,16 @@ Usage
 
 To use this library do you need `Anticaptcha.com`_ API key.
 
+You can pass the key explicitly or set the ``ANTICAPTCHA_API_KEY`` environment variable:
+
+.. code:: python
+
+    # Explicit key
+    client = AnticaptchaClient("my-api-key")
+
+    # Or set ANTICAPTCHA_API_KEY environment variable
+    client = AnticaptchaClient()
+
 Solve recaptcha
 ###############
 
diff --git a/python_anticaptcha/base.py b/python_anticaptcha/base.py
index a5188f7..a0014e7 100644
--- a/python_anticaptcha/base.py
+++ b/python_anticaptcha/base.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import os
 import requests
 import time
 import json
@@ -89,9 +90,15 @@ class AnticaptchaClient:
     response_timeout = 5
 
     def __init__(
-        self, client_key: str, language_pool: str = "en", host: str = "api.anti-captcha.com", use_ssl: bool = True,
+        self, client_key: str | None = None, language_pool: str = "en", host: str = "api.anti-captcha.com", use_ssl: bool = True,
     ) -> None:
-        self.client_key = client_key
+        self.client_key = client_key or os.environ.get("ANTICAPTCHA_API_KEY")
+        if not self.client_key:
+            raise AnticaptchaException(
+                None,
+                "CONFIG_ERROR",
+                "API key required. Pass client_key or set ANTICAPTCHA_API_KEY env var.",
+            )
         self.language_pool = language_pool
         self.base_url = "{proto}://{host}/".format(
             proto="https" if use_ssl else "http", host=host
diff --git a/tests/test_base.py b/tests/test_base.py
index 0582aad..2794b85 100644
--- a/tests/test_base.py
+++ b/tests/test_base.py
@@ -1,4 +1,5 @@
 from unittest.mock import patch, MagicMock
+import os
 import pytest
 
 from python_anticaptcha.base import AnticaptchaClient, Job, SLEEP_EVERY_CHECK_FINISHED
@@ -23,6 +24,22 @@ def test_language_pool(self):
         client = AnticaptchaClient("key123", language_pool="rn")
         assert client.language_pool == "rn"
 
+    def test_env_var_fallback(self, monkeypatch):
+        monkeypatch.setenv("ANTICAPTCHA_API_KEY", "env-key-456")
+        client = AnticaptchaClient()
+        assert client.client_key == "env-key-456"
+
+    def test_explicit_key_over_env(self, monkeypatch):
+        monkeypatch.setenv("ANTICAPTCHA_API_KEY", "env-key-456")
+        client = AnticaptchaClient("explicit-key-789")
+        assert client.client_key == "explicit-key-789"
+
+    def test_no_key_raises(self, monkeypatch):
+        monkeypatch.delenv("ANTICAPTCHA_API_KEY", raising=False)
+        with pytest.raises(AnticaptchaException) as exc_info:
+            AnticaptchaClient()
+        assert exc_info.value.error_code == "CONFIG_ERROR"
+
 
 class TestCheckResponse:
     def setup_method(self):