diff --git a/pyproject.toml b/pyproject.toml
index c151100a..414e83c2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,7 +53,10 @@ test = [
     "flake8-debugger==4.1.2",
     "flake8-imports==0.1.1",
 ]
-async = ["httpx>=0.15.0"]
+async = [
+    "httpx>=0.15.0",
+    "packaging",
+]
 xmlsec = ["xmlsec>=0.6.1"]
 
 [build-system]
diff --git a/src/zeep/transports.py b/src/zeep/transports.py
index 2a1ee8bd..d2136373 100644
--- a/src/zeep/transports.py
+++ b/src/zeep/transports.py
@@ -16,6 +16,15 @@
 except ImportError:
     httpx = None
 
+try:
+    from packaging.version import Version
+    if httpx is None or Version(httpx.__version__) < Version("0.26.0"):
+        HTTPX_PROXY_KWARG_NAME = "proxies"
+    else:
+        HTTPX_PROXY_KWARG_NAME = "proxy"
+except ImportError:
+    Version = None
+    HTTPX_PROXY_KWARG_NAME = None
 
 __all__ = ["AsyncTransport", "Transport"]
 
@@ -178,20 +187,24 @@ def __init__(
         verify_ssl=True,
         proxy=None,
     ):
-        if httpx is None:
-            raise RuntimeError("The AsyncTransport is based on the httpx module")
+        if httpx is None or HTTPX_PROXY_KWARG_NAME is None:
+            raise RuntimeError(
+                "To use AsyncTransport, install zeep with the async extras, "
+                "e.g., `pip install zeep[async]`"
+            )
 
         self._close_session = False
         self.cache = cache
+        proxy_kwargs = {HTTPX_PROXY_KWARG_NAME: proxy}
         self.wsdl_client = wsdl_client or httpx.Client(
             verify=verify_ssl,
-            proxies=proxy,
             timeout=timeout,
+            **proxy_kwargs,
         )
         self.client = client or httpx.AsyncClient(
             verify=verify_ssl,
-            proxies=proxy,
             timeout=operation_timeout,
+            **proxy_kwargs,
         )
         self.logger = logging.getLogger(__name__)