diff --git a/lib/mint/web_socket.ex b/lib/mint/web_socket.ex
index f1a01c45..01839afb 100644
--- a/lib/mint/web_socket.ex
+++ b/lib/mint/web_socket.ex
@@ -208,6 +208,11 @@ defmodule Mint.WebSocket do
   connections established by passing `:http` to `Mint.HTTP.connect/4` and
   `:wss` corresponding to `:https`.
 
+  The WebSocket handshake headers are subject to the `:case_sensitive_headers`
+  option of `Mint.HTTP.connect/4`: they are lowercased by default and sent in
+  their conventional casing (`Sec-WebSocket-Key` and friends) when that option
+  is enabled.
+
   ## Options
 
     * `:extensions` - a list of extensions to negotiate. See the extensions
diff --git a/lib/mint/web_socket/utils.ex b/lib/mint/web_socket/utils.ex
index 3a8eb913..42cdc0b8 100644
--- a/lib/mint/web_socket/utils.ex
+++ b/lib/mint/web_socket/utils.ex
@@ -9,21 +9,23 @@ defmodule Mint.WebSocket.Utils do
     :crypto.strong_rand_bytes(16) |> Base.encode64()
   end
 
+  # header names are written in their conventional casing: Mint lowercases them
+  # unless the connection was opened with `case_sensitive_headers: true`
   def headers({:http1, nonce}, extensions) when is_binary(nonce) do
     [
-      {"upgrade", "websocket"},
-      {"connection", "upgrade"},
-      {"sec-websocket-version", "13"},
-      {"sec-websocket-key", nonce},
-      {"sec-websocket-extensions", extension_string(extensions)}
+      {"Upgrade", "websocket"},
+      {"Connection", "upgrade"},
+      {"Sec-WebSocket-Version", "13"},
+      {"Sec-WebSocket-Key", nonce},
+      {"Sec-WebSocket-Extensions", extension_string(extensions)}
     ]
     |> Enum.reject(fn {_k, v} -> v == "" end)
   end
 
   def headers(:http2, extensions) do
     [
-      {"sec-websocket-version", "13"},
-      {"sec-websocket-extensions", extension_string(extensions)}
+      {"Sec-WebSocket-Version", "13"},
+      {"Sec-WebSocket-Extensions", extension_string(extensions)}
     ]
     |> Enum.reject(fn {_k, v} -> v == "" end)
   end
diff --git a/test/mint/web_socket_test.exs b/test/mint/web_socket_test.exs
index 12f72167..fdc9ed9f 100644
--- a/test/mint/web_socket_test.exs
+++ b/test/mint/web_socket_test.exs
@@ -119,6 +119,44 @@ defmodule Mint.WebSocketTest do
     end
   end
 
+  describe "given an HTTP/1 connection to a socket which captures the request" do
+    setup do
+      {:ok, listen_socket} = :gen_tcp.listen(0, [:binary, active: false, reuseaddr: true])
+      on_exit(fn -> :gen_tcp.close(listen_socket) end)
+      {:ok, port} = :inet.port(listen_socket)
+
+      [listen_socket: listen_socket, port: port]
+    end
+
+    test "the upgrade headers are lowercased by default", c do
+      {:ok, conn} = HTTP1.connect(:http, "localhost", c.port)
+      {:ok, _conn, _ref} = WebSocket.upgrade(:ws, conn, "/", [])
+
+      request = capture_request(c.listen_socket)
+      assert request =~ "upgrade: websocket"
+      assert request =~ "sec-websocket-key: "
+    end
+
+    test "the upgrade headers keep their casing with case_sensitive_headers: true", c do
+      {:ok, conn} = HTTP1.connect(:http, "localhost", c.port, case_sensitive_headers: true)
+      {:ok, _conn, _ref} = WebSocket.upgrade(:ws, conn, "/", [])
+
+      request = capture_request(c.listen_socket)
+      assert request =~ "Upgrade: websocket"
+      assert request =~ "Connection: upgrade"
+      assert request =~ "Sec-WebSocket-Version: 13"
+      assert request =~ "Sec-WebSocket-Key: "
+    end
+  end
+
+  defp capture_request(listen_socket) do
+    {:ok, socket} = :gen_tcp.accept(listen_socket, 5_000)
+    {:ok, request} = :gen_tcp.recv(socket, 0, 5_000)
+    :gen_tcp.close(socket)
+
+    request
+  end
+
   @doc !"""
        In Mint 1.5.0+, Mint handles the SETTINGS frame from the server asynchronously
        and returns default values for server settings until it is received. So we must