diff --git a/starlette/middleware/trustedhost.py b/starlette/middleware/trustedhost.py
index 5f733dc1..536a1968 100644
--- a/starlette/middleware/trustedhost.py
+++ b/starlette/middleware/trustedhost.py
@@ -1,7 +1,7 @@
 import typing
 
-from starlette.datastructures import Headers
-from starlette.responses import PlainTextResponse
+from starlette.datastructures import URL, Headers
+from starlette.responses import PlainTextResponse, RedirectResponse
 from starlette.types import ASGIApp, ASGIInstance, Scope
 
 ENFORCE_DOMAIN_WILDCARD = "Domain wildcard patterns must be like '*.example.com'."
@@ -9,28 +9,38 @@ ENFORCE_DOMAIN_WILDCARD = "Domain wildcard patterns must be like '*.example.com'
 
 class TrustedHostMiddleware:
     def __init__(
-        self, app: ASGIApp, allowed_hosts: typing.Sequence[str] = ["*"]
+        self,
+        app: ASGIApp,
+        allowed_hosts: typing.Sequence[str] = ["*"],
+        www_redirect: bool = True,
     ) -> None:
         for pattern in allowed_hosts:
             assert "*" not in pattern[1:], ENFORCE_DOMAIN_WILDCARD
             if pattern.startswith("*") and pattern != "*":
                 assert pattern.startswith("*."), ENFORCE_DOMAIN_WILDCARD
         self.app = app
-        self.allowed_hosts = allowed_hosts
+        self.allowed_hosts = list(allowed_hosts)
         self.allow_any = "*" in allowed_hosts
+        self.www_redirect = www_redirect
 
     def __call__(self, scope: Scope) -> ASGIInstance:
         if scope["type"] in ("http", "websocket") and not self.allow_any:
             headers = Headers(scope=scope)
             host = headers.get("host", "").split(":")[0]
+            found_www_redirect = False
             for pattern in self.allowed_hosts:
-                if (
-                    host == pattern
-                    or pattern.startswith("*")
-                    and host.endswith(pattern[1:])
+                if host == pattern or (
+                    pattern.startswith("*") and host.endswith(pattern[1:])
                 ):
                     break
+                elif "www." + host == pattern:
+                    found_www_redirect = True
             else:
+                if found_www_redirect and self.www_redirect:
+                    url = URL(scope=scope)
+                    redirect_url = url.replace(netloc="www." + url.netloc)
+                    print(redirect_url)
+                    return RedirectResponse(url=str(redirect_url))
                 return PlainTextResponse("Invalid host header", status_code=400)
 
         return self.app(scope)
diff --git a/tests/middleware/test_trusted_host.py b/tests/middleware/test_trusted_host.py
index d8127d3e..54b2a0c1 100644
--- a/tests/middleware/test_trusted_host.py
+++ b/tests/middleware/test_trusted_host.py
@@ -26,3 +26,18 @@ def test_trusted_host_middleware():
     client = TestClient(app, base_url="http://invalidhost")
     response = client.get("/")
     assert response.status_code == 400
+
+
+def test_www_redirect():
+    app = Starlette()
+
+    app.add_middleware(TrustedHostMiddleware, allowed_hosts=["www.example.com"])
+
+    @app.route("/")
+    def homepage(request):
+        return PlainTextResponse("OK", status_code=200)
+
+    client = TestClient(app, base_url="https://example.com")
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.url == "https://www.example.com/"