diff --git a/src/client.toit b/src/client.toit
index ba95517..51fea12 100644
--- a/src/client.toit
+++ b/src/client.toit
@@ -102,6 +102,7 @@ class Client:
   interface_/tcp.Interface
 
   use-tls-by-default_ ::= false
+  allow-insecure-redirects_/bool
   certificate_/tls.Certificate? ::= null
   server-name_/string? ::= null
   root-certificates_/List ::= []
@@ -137,14 +138,23 @@ class Client:
     buffer memory for the TLS connection.  Call $close (perhaps in a finally
     clause) to release the connection.
 
+  By default, following an HTTPS-to-HTTP or WSS-to-WS redirect throws
+    `INSECURE_REDIRECT` before connecting to the insecure destination. This
+    applies at every hop, including redirects that switch from POST to GET.
+    Set $allow-insecure-redirects to true to permit these redirects, which may
+    expose request headers and bodies over plaintext. Explicit HTTP or WS
+    requests and requests that do not follow redirects are unaffected.
+
   See the `certificate_roots` package for common roots:
     https://pkg.toit.io/package/github.com%2Ftoitware%2Ftoit-cert-roots
   */
   constructor network/tcp.Interface
       --root-certificates/List=[]
-      --security-store/SecurityStore=SecurityStoreInMemory:
+      --security-store/SecurityStore=SecurityStoreInMemory
+      --allow-insecure-redirects/bool=false:
     interface_ = network
     security-store_ = security-store
+    allow-insecure-redirects_ = allow-insecure-redirects
     root-certificates_ = root-certificates
     add-finalizer this:: this.finalize_
 
@@ -159,13 +169,17 @@ class Client:
   The $server-name can be specified for verifying the TLS certificate.  This is
     for the rare case where we wish to verify the TLS connections with a
     different server name from the one used to establish the connection.
+
+  The $allow-insecure-redirects option has the same meaning as in $constructor.
   */
   constructor.tls .interface_
       --root-certificates/List=[]
       --server-name/string?=null
       --certificate/tls.Certificate?=null
-      --security-store/SecurityStore=SecurityStoreInMemory:
+      --security-store/SecurityStore=SecurityStoreInMemory
+      --allow-insecure-redirects/bool=false:
     security-store_ = security-store
+    allow-insecure-redirects_ = allow-insecure-redirects
     use-tls-by-default_ = true
     root-certificates_ = root-certificates
     server-name_ = server-name
@@ -441,7 +455,10 @@ class Client:
 
   get-location_ response/Response previous/ParsedUri_ -> ParsedUri_:
     location := response.headers.single "Location"
-    return ParsedUri_.parse_ location --previous=previous
+    next := ParsedUri_.parse_ location --previous=previous
+    if previous.use-tls and not next.use-tls and not allow-insecure-redirects_:
+      throw "INSECURE_REDIRECT"
+    return next
 
   redirect-headers_ headers/Headers? previous/ParsedUri_ next/ParsedUri_ -> Headers?:
     if not headers or next.can-reuse-connection previous: return headers
diff --git a/tests/client-redirect-downgrade-test.toit b/tests/client-redirect-downgrade-test.toit
new file mode 100644
index 0000000..456e479
--- /dev/null
+++ b/tests/client-redirect-downgrade-test.toit
@@ -0,0 +1,103 @@
+// Copyright (C) 2026 Toit contributors.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the tests/TESTS_LICENSE file.
+
+import expect show *
+import http
+import http.connection show Connection
+
+import .memory-socket
+
+OK ::= "HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n"
+
+main:
+  [false, true].do: | tls-client/bool |
+    ["GET", "POST", "JSON", "FORM", "WebSocket"].do: | method/string |
+      [301, 302, 303, 307, 308].do: | status/int |
+        [false, true].do: | allow/bool |
+          [false, true].do: | chain/bool |
+            test-redirect method status --tls-client=tls-client --allow=allow --chain=chain
+        test-redirect method status --tls-client=tls-client --no-follow
+    test-explicit-http --tls-client=tls-client
+  test-secure-redirect
+
+redirect status/int location/string -> string:
+  return "HTTP/1.1 $status Redirect\r\nLocation: $location\r\nContent-Length: 0\r\n\r\n"
+
+test-redirect method/string status/int
+    --tls-client/bool --allow/bool=false --chain/bool=false --follow/bool=true:
+  web-socket := method == "WebSocket"
+  secure-scheme := web-socket ? "wss" : "https"
+  insecure-scheme := web-socket ? "ws" : "http"
+  uri := "$secure-scheme://original.example/"
+  initial-response := redirect status "$insecure-scheme://other.example/target"
+  if chain: initial-response = (redirect 307 "/intermediate") + initial-response
+  // Model an established secure connection. The test exercises redirect
+  // handling and wire output without certificates or a live TLS server.
+  first := MemorySocket [initial-response.to-byte-array]
+  second := MemorySocket [OK.to-byte-array]
+  network := ScriptedNetwork [second]
+  client := tls-client ? (http.Client.tls network) : (http.Client network)
+  if allow:
+    client.close
+    client = tls-client
+        ? (http.Client.tls network --allow-insecure-redirects)
+        : (http.Client network --allow-insecure-redirects)
+  parsed := http.ParsedUri_.parse_ uri
+  client.connection_ = Connection first --location=parsed --host=parsed.host-with-port
+  try:
+    error := catch:
+      if web-socket:
+        client.web-socket --uri=uri --follow-redirects=follow
+      else:
+        response := null
+        if method == "GET":
+          response = client.get --uri=uri --follow-redirects=follow
+        else if method == "POST":
+          response = client.post "synthetic-secret".to-byte-array --uri=uri --follow-redirects=follow
+        else if method == "JSON":
+          response = client.post-json {"secret": "synthetic-secret"} --uri=uri --follow-redirects=follow
+        else:
+          response = client.post-form {"secret": "synthetic-secret"} --uri=uri --follow-redirects=follow
+        expect-equals (follow ? 200 : status) response.status-code
+    if not follow:
+      expect-equals (web-socket ? "WebSocket upgrade failed with $status Redirect" : null) error
+    else if not allow:
+      expect-equals "INSECURE_REDIRECT" error
+    else:
+      expect-equals (web-socket ? "WebSocket upgrade failed with 200 OK" : null) error
+    expect-equals (follow and allow ? 1 : 0) network.index_
+    sent := second.out.buffer.to-string
+    if follow and allow:
+      redirected-method := (method == "GET" or web-socket or status == 303) ? "GET" : "POST"
+      expect (sent.starts-with "$redirected-method /target HTTP/1.1\r\n")
+      expect-equals (redirected-method == "POST") (sent.contains "synthetic-secret")
+    else:
+      expect-equals "" sent
+    if chain and follow:
+      expect (first.out.buffer.to-string.contains " /intermediate HTTP/1.1\r\n")
+  finally:
+    client.close
+
+test-explicit-http --tls-client/bool:
+  socket := MemorySocket [OK.to-byte-array]
+  network := ScriptedNetwork [socket]
+  client := tls-client ? (http.Client.tls network) : (http.Client network)
+  try:
+    response := client.get --uri="http://original.example/"
+    expect-equals 200 response.status-code
+  finally:
+    client.close
+
+test-secure-redirect:
+  // A same-origin HTTPS redirect remains allowed with the default policy.
+  socket := MemorySocket [((redirect 307 "/next") + OK).to-byte-array]
+  client := http.Client (ScriptedNetwork [])
+  parsed := http.ParsedUri_.parse_ "https://original.example/"
+  client.connection_ = Connection socket --location=parsed --host=parsed.host-with-port
+  try:
+    response := client.post "synthetic-secret".to-byte-array --uri=parsed.stringify
+    expect-equals 200 response.status-code
+    expect (socket.out.buffer.to-string.contains "POST /next HTTP/1.1\r\n")
+  finally:
+    client.close