diff --git a/integration_test/mix.lock b/integration_test/mix.lock
index 83ecfe3153..3276bfe7e3 100644
--- a/integration_test/mix.lock
+++ b/integration_test/mix.lock
@@ -47,4 +47,6 @@
   "telemetry": {:hex, :telemetry, "1.1.0", "a589817034a27eab11144ad24d5c0f9fab1f58173274b1e9bae7074af9cbee51", [:rebar3], [], "hexpm", "b727b2a1f75614774cff2d7565b64d0dfa5bd52ba517f16543e6fc7efcc0df48"},
   "telemetry_metrics": {:hex, :telemetry_metrics, "0.6.1", "315d9163a1d4660aedc3fee73f33f1d355dcc76c5c3ab3d59e76e3edf80eef1f", [:mix], [{:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "7be9e0871c41732c233be71e4be11b96e56177bf15dde64a8ac9ce72ac9834c6"},
   "telemetry_poller": {:hex, :telemetry_poller, "1.0.0", "db91bb424e07f2bb6e73926fcafbfcbcb295f0193e0a00e825e589a0a47e8453", [:rebar3], [{:telemetry, "~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "b3a24eafd66c3f42da30fc3ca7dda1e9d546c12250a2d60d7b81d264fbec4f6e"},
+  "websock": {:hex, :websock, "0.4.3", "184ac396bdcd3dfceb5b74c17d221af659dd559a95b1b92041ecb51c9b728093", [:mix], [], "hexpm", "5e4dd85f305f43fd3d3e25d70bec4a45228dfed60f0f3b072d8eddff335539cf"},
+  "websock_adapter": {:hex, :websock_adapter, "0.4.3", "30dd07a9fabf704845baf0176b3e8508d3468539422ffe9ff4229fa39b6277c9", [:mix], [{:bandit, "~> 0.5.9", [hex: :bandit, repo: "hexpm", optional: true]}, {:plug, "~> 1.14.0", [hex: :plug, repo: "hexpm", optional: false]}, {:plug_cowboy, "~> 2.6", [hex: :plug_cowboy, repo: "hexpm", optional: true]}, {:websock, "~> 0.4.3", [hex: :websock, repo: "hexpm", optional: false]}], "hexpm", "304bf61322080bc0e6ea341263b63097f0c801af4638a86f264287ea8fd3a9ff"},
 }
diff --git a/lib/phoenix/endpoint/cowboy2_adapter.ex b/lib/phoenix/endpoint/cowboy2_adapter.ex
index 3f324bfa31..84117a9c7a 100644
--- a/lib/phoenix/endpoint/cowboy2_adapter.ex
+++ b/lib/phoenix/endpoint/cowboy2_adapter.ex
@@ -137,26 +137,4 @@ defmodule Phoenix.Endpoint.Cowboy2Adapter do
   defp port_to_integer({:system, env_var}), do: port_to_integer(System.get_env(env_var))
   defp port_to_integer(port) when is_binary(port), do: String.to_integer(port)
   defp port_to_integer(port) when is_integer(port), do: port
-
-  @doc false
-  def websocket_upgrade(conn, handler, state, opts) do
-    cowboy_opts =
-      opts
-      |> Enum.flat_map(fn
-        {:timeout, timeout} -> [idle_timeout: timeout]
-        {:compress, _} = opt -> [opt]
-        {:max_frame_size, _} = opt -> [opt]
-        _other -> []
-      end)
-      |> Map.new()
-
-    process_flags =
-      opts
-      |> Keyword.take([:fullsweep_after])
-      |> Map.new()
-
-    handler_args = {handler, process_flags, state}
-    upgrade_args = {Phoenix.Endpoint.Cowboy2Handler, handler_args, cowboy_opts}
-    Plug.Conn.upgrade_adapter(conn, :websocket, upgrade_args)
-  end
 end
diff --git a/lib/phoenix/endpoint/cowboy2_handler.ex b/lib/phoenix/endpoint/cowboy2_handler.ex
deleted file mode 100644
index c3c97852d1..0000000000
--- a/lib/phoenix/endpoint/cowboy2_handler.ex
+++ /dev/null
@@ -1,83 +0,0 @@
-defmodule Phoenix.Endpoint.Cowboy2Handler do
-  @moduledoc false
-
-  @behaviour :cowboy_websocket
-
-  # We never actually call this; it's just here to quell compiler warnings
-  @impl true
-  def init(req, state), do: {:cowboy_websocket, req, state}
-
-  defp handle_reply(handler, {:ok, state}), do: {:ok, [handler | state]}
-  defp handle_reply(handler, {:push, data, state}), do: {:reply, data, [handler | state]}
-
-  defp handle_reply(handler, {:reply, _status, data, state}),
-    do: {:reply, data, [handler | state]}
-
-  defp handle_reply(handler, {:stop, _reason, state}), do: {:stop, [handler | state]}
-
-  defp handle_control_frame(payload_with_opts, handler_state) do
-    [handler | state] = handler_state
-    reply =
-      if function_exported?(handler, :handle_control, 2) do
-        handler.handle_control(payload_with_opts, state)
-      else
-        {:ok, state}
-      end
-
-    handle_reply(handler, reply)
-  end
-
-  @impl true
-  def websocket_init({handler, process_flags, state}) do
-    for {key, value} <- process_flags do
-      :erlang.process_flag(key, value)
-    end
-
-    {:ok, state} = handler.init(state)
-    {:ok, [handler | state]}
-  end
-
-  @impl true
-  def websocket_handle({opcode, payload}, [handler | state]) when opcode in [:text, :binary] do
-    handle_reply(handler, handler.handle_in({payload, opcode: opcode}, state))
-  end
-
-  def websocket_handle({opcode, payload}, handler_state) when opcode in [:ping, :pong] do
-    handle_control_frame({payload, opcode: opcode}, handler_state)
-  end
-
-  def websocket_handle(opcode, handler_state) when opcode in [:ping, :pong] do
-    handle_control_frame({nil, opcode: opcode}, handler_state)
-  end
-
-  def websocket_handle(_other, handler_state) do
-    {:ok, handler_state}
-  end
-
-  @impl true
-  def websocket_info(message, [handler | state]) do
-    handle_reply(handler, handler.handle_info(message, state))
-  end
-
-  @impl true
-  def terminate({:error, :closed}, _req, [handler | state]) do
-    handler.terminate(:closed, state)
-  end
-
-  def terminate({:remote, :closed}, _req, [handler | state]) do
-    handler.terminate(:closed, state)
-  end
-
-  def terminate({:remote, code, _}, _req, [handler | state])
-      when code in 1000..1003 or code in 1005..1011 or code == 1015 do
-    handler.terminate(:closed, state)
-  end
-
-  def terminate(:remote, _req, [handler | state]) do
-    handler.terminate(:closed, state)
-  end
-
-  def terminate(reason, _req, [handler | state]) do
-    handler.terminate(reason, state)
-  end
-end
diff --git a/lib/phoenix/transports/websocket.ex b/lib/phoenix/transports/websocket.ex
index 118949139d..16be35bb54 100644
--- a/lib/phoenix/transports/websocket.ex
+++ b/lib/phoenix/transports/websocket.ex
@@ -1,5 +1,18 @@
 defmodule Phoenix.Transports.WebSocket do
   @moduledoc false
+  #
+  # How WebSockets Work In Phoenix
+  #
+  # WebSocket support in Phoenix is implemented on top of the `WebSockAdapter` library. Upgrade
+  # requests from clients originate as regular HTTP requests that get routed to this module via
+  # Plug. These requests are then upgraded to WebSocket connections via
+  # `WebSockAdapter.upgrade/4`, which takes as an argument the handler for a given socket endpoint
+  # as configured in the application's Endpoint. This handler module must implement the
+  # transport-agnostic `Phoenix.Socket.Transport` behaviour (this same behaviour is also used for
+  # other transports such as long polling). Because this behaviour is a superset of the `WebSock`
+  # behaviour, the `WebSock` library is able to use the callbacks in the `WebSock` behaviour to
+  # call this handler module directly for the rest of the WebSocket connection's lifetime.
+  #
   @behaviour Plug
 
   import Plug.Conn
@@ -44,12 +57,8 @@ defmodule Phoenix.Transports.WebSocket do
 
         case handler.connect(config) do
           {:ok, arg} ->
-            upgrade =
-              conn.private[:phoenix_websocket_upgrade] ||
-                (&Phoenix.Endpoint.Cowboy2Adapter.websocket_upgrade/4)
-
             conn
-            |> upgrade.(handler, arg, opts)
+            |> WebSockAdapter.upgrade(handler, arg, opts)
             |> halt()
 
           :error ->
diff --git a/mix.exs b/mix.exs
index ca7923691f..2c832e7923 100644
--- a/mix.exs
+++ b/mix.exs
@@ -72,9 +72,9 @@ defmodule Phoenix.MixProject do
       {:plug_crypto, "~> 1.2"},
       {:telemetry, "~> 0.4 or ~> 1.0"},
       {:phoenix_pubsub, "~> 2.1"},
-
       {:phoenix_view, "~> 2.0", optional: true},
       {:phoenix_template, "~> 1.0"},
+      {:websock_adapter, "~> 0.4"},
 
       # TODO drop castore when we require OTP 25+
       {:castore, ">= 0.0.0"},
diff --git a/mix.lock b/mix.lock
index 1082c3224d..4a71a6e939 100644
--- a/mix.lock
+++ b/mix.lock
@@ -34,4 +34,6 @@
   "telemetry": {:hex, :telemetry, "1.1.0", "a589817034a27eab11144ad24d5c0f9fab1f58173274b1e9bae7074af9cbee51", [:rebar3], [], "hexpm", "b727b2a1f75614774cff2d7565b64d0dfa5bd52ba517f16543e6fc7efcc0df48"},
   "telemetry_metrics": {:hex, :telemetry_metrics, "0.6.1", "315d9163a1d4660aedc3fee73f33f1d355dcc76c5c3ab3d59e76e3edf80eef1f", [:mix], [{:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "7be9e0871c41732c233be71e4be11b96e56177bf15dde64a8ac9ce72ac9834c6"},
   "telemetry_poller": {:hex, :telemetry_poller, "1.0.0", "db91bb424e07f2bb6e73926fcafbfcbcb295f0193e0a00e825e589a0a47e8453", [:rebar3], [{:telemetry, "~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "b3a24eafd66c3f42da30fc3ca7dda1e9d546c12250a2d60d7b81d264fbec4f6e"},
+  "websock": {:hex, :websock, "0.4.3", "184ac396bdcd3dfceb5b74c17d221af659dd559a95b1b92041ecb51c9b728093", [:mix], [], "hexpm", "5e4dd85f305f43fd3d3e25d70bec4a45228dfed60f0f3b072d8eddff335539cf"},
+  "websock_adapter": {:hex, :websock_adapter, "0.4.3", "30dd07a9fabf704845baf0176b3e8508d3468539422ffe9ff4229fa39b6277c9", [:mix], [{:bandit, "~> 0.5.9", [hex: :bandit, repo: "hexpm", optional: true]}, {:plug, "~> 1.14.0", [hex: :plug, repo: "hexpm", optional: false]}, {:plug_cowboy, "~> 2.6", [hex: :plug_cowboy, repo: "hexpm", optional: true]}, {:websock, "~> 0.4.3", [hex: :websock, repo: "hexpm", optional: false]}], "hexpm", "304bf61322080bc0e6ea341263b63097f0c801af4638a86f264287ea8fd3a9ff"},
 }