diff --git a/README.md b/README.md
index 3d607542..836dc982 100644
--- a/README.md
+++ b/README.md
@@ -360,6 +360,7 @@ import httpx
 from anthropic import Anthropic
 
 client = Anthropic(
+    # Or use the `ANTHROPIC_BASE_URL` env var
     base_url="http://my.test.server.example.com:8083",
     http_client=httpx.Client(
         proxies="http://my.test.proxy.example.com",
diff --git a/src/anthropic/_client.py b/src/anthropic/_client.py
index 62f80393..ab7a5cc6 100644
--- a/src/anthropic/_client.py
+++ b/src/anthropic/_client.py
@@ -103,6 +103,8 @@ def __init__(
             auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN")
         self.auth_token = auth_token
 
+        if base_url is None:
+            base_url = os.environ.get("ANTHROPIC_BASE_URL")
         if base_url is None:
             base_url = f"https://api.anthropic.com"
 
@@ -362,6 +364,8 @@ def __init__(
             auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN")
         self.auth_token = auth_token
 
+        if base_url is None:
+            base_url = os.environ.get("ANTHROPIC_BASE_URL")
         if base_url is None:
             base_url = f"https://api.anthropic.com"
 
diff --git a/tests/test_client.py b/tests/test_client.py
index 8cfa59ab..7d03435d 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -27,6 +27,8 @@
     make_request_options,
 )
 
+from .utils import update_env
+
 base_url = os.environ.get("TEST_API_BASE_URL", "http://127.0.0.1:4010")
 api_key = "my-anthropic-api-key"
 
@@ -408,6 +410,11 @@ class Model2(BaseModel):
         assert isinstance(response, Model1)
         assert response.foo == 1
 
+    def test_base_url_env(self) -> None:
+        with update_env(ANTHROPIC_BASE_URL="http://localhost:5000/from/env"):
+            client = Anthropic(api_key=api_key, _strict_response_validation=True)
+            assert client.base_url == "http://localhost:5000/from/env/"
+
     @pytest.mark.parametrize(
         "client",
         [
@@ -1036,6 +1043,11 @@ class Model2(BaseModel):
         assert isinstance(response, Model1)
         assert response.foo == 1
 
+    def test_base_url_env(self) -> None:
+        with update_env(ANTHROPIC_BASE_URL="http://localhost:5000/from/env"):
+            client = AsyncAnthropic(api_key=api_key, _strict_response_validation=True)
+            assert client.base_url == "http://localhost:5000/from/env/"
+
     @pytest.mark.parametrize(
         "client",
         [
diff --git a/tests/utils.py b/tests/utils.py
index 0a1733f6..348363a5 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -1,7 +1,9 @@
 from __future__ import annotations
 
+import os
 import traceback
-from typing import Any, TypeVar, cast
+import contextlib
+from typing import Any, TypeVar, Iterator, cast
 from datetime import date, datetime
 from typing_extensions import Literal, get_args, get_origin, assert_type
 
@@ -103,3 +105,16 @@ def _assert_list_type(type_: type[object], value: object) -> None:
     inner_type = get_args(type_)[0]
     for entry in value:
         assert_type(inner_type, entry)  # type: ignore
+
+
+@contextlib.contextmanager
+def update_env(**new_env: str) -> Iterator[None]:
+    old = os.environ.copy()
+
+    try:
+        os.environ.update(new_env)
+
+        yield None
+    finally:
+        os.environ.clear()
+        os.environ.update(old)