diff --git a/lib/plug/adapters/test/conn.ex b/lib/plug/adapters/test/conn.ex
index e396ceb3..c3315ff1 100644
--- a/lib/plug/adapters/test/conn.ex
+++ b/lib/plug/adapters/test/conn.ex
@@ -1,5 +1,6 @@
 defmodule Plug.Adapters.Test.Conn do
   @behaviour Plug.Conn.Adapter
+  @already_sent Plug.Conn.Adapter.already_sent()
   @moduledoc false
 
   ## Test helpers
@@ -43,7 +44,6 @@ defmodule Plug.Adapters.Test.Conn do
       | adapter: {__MODULE__, state},
         host: uri.host || conn.host || "www.example.com",
         method: method,
-        owner: owner,
         path_info: split_path(uri.path),
         port: uri.port || conn_port,
         remote_ip: conn.remote_ip || {127, 0, 0, 1},
@@ -88,7 +88,10 @@ defmodule Plug.Adapters.Test.Conn do
     do_send(state, status, headers, data)
   end
 
-  def send_chunked(state, _status, _headers), do: {:ok, "", %{state | chunks: ""}}
+  def send_chunked(%{owner: owner} = state, _status, _headers) do
+    send(owner, @already_sent)
+    {:ok, "", %{state | chunks: ""}}
+  end
 
   def chunk(%{method: "HEAD"} = state, _body), do: {:ok, "", state}
 
@@ -98,6 +101,7 @@ defmodule Plug.Adapters.Test.Conn do
   end
 
   defp do_send(%{owner: owner, ref: ref} = state, status, headers, body) do
+    send(owner, @already_sent)
     send(owner, {ref, {status, headers, body}})
     {:ok, body, state}
   end
diff --git a/lib/plug/conn.ex b/lib/plug/conn.ex
index b163ed1a..b0f8221f 100644
--- a/lib/plug/conn.ex
+++ b/lib/plug/conn.ex
@@ -174,7 +174,6 @@ defmodule Plug.Conn do
   @type headers :: [{binary, binary}]
   @type host :: binary
   @type int_status :: non_neg_integer | nil
-  @type owner :: pid
   @type method :: binary
   @type query_param :: binary | %{optional(binary) => query_param} | [query_param]
   @type query_params :: %{optional(binary) => query_param}
@@ -196,7 +195,7 @@ defmodule Plug.Conn do
           halted: halted,
           host: host,
           method: method,
-          owner: owner,
+          owner: pid | nil,
           params: params | Unfetched.t(),
           path_info: segments,
           path_params: query_params,
@@ -447,7 +446,7 @@ defmodule Plug.Conn do
     {:ok, body, payload} =
       adapter.send_resp(payload, conn.status, conn.resp_headers, conn.resp_body)
 
-    send(owner, @already_sent)
+    owner && send(owner, @already_sent)
     %{conn | adapter: {adapter, payload}, resp_body: body, state: :sent}
   end
 
@@ -498,7 +497,7 @@ defmodule Plug.Conn do
     {:ok, body, payload} =
       adapter.send_file(payload, conn.status, conn.resp_headers, file, offset, length)
 
-    send(owner, @already_sent)
+    owner && send(owner, @already_sent)
     %{conn | adapter: {adapter, payload}, state: :file, resp_body: body}
   end
 
@@ -526,7 +525,7 @@ defmodule Plug.Conn do
     conn = %{conn | status: Plug.Conn.Status.code(status), resp_body: nil}
     conn = run_before_send(conn, :set_chunked)
     {:ok, body, payload} = adapter.send_chunked(payload, conn.status, conn.resp_headers)
-    send(owner, @already_sent)
+    owner && send(owner, @already_sent)
     %{conn | adapter: {adapter, payload}, state: :chunked, resp_body: body}
   end
 
diff --git a/lib/plug/conn/adapter.ex b/lib/plug/conn/adapter.ex
index bf7780aa..280f0ce8 100644
--- a/lib/plug/conn/adapter.ex
+++ b/lib/plug/conn/adapter.ex
@@ -1,6 +1,13 @@
 defmodule Plug.Conn.Adapter do
   @moduledoc """
   Specification of the connection adapter API implemented by webservers.
+
+  ## Implementation recommendations
+
+  The `owner` field of `Plug.Conn` is deprecated and no longer needs to
+  be set by adapters. If you don't set the `owner` field, it is the
+  responsibility of the adapters to track the owner and send the
+  `already_sent/0` message below on any of the `send_*` callbacks.
   """
   alias Plug.Conn
 
@@ -12,6 +19,13 @@ defmodule Plug.Conn.Adapter do
           ssl_cert: binary | nil
         }
 
+  @doc """
+  The message to send to the request process on send callbacks.
+  """
+  def already_sent do
+    {:plug_conn, :sent}
+  end
+
   @doc """
   Function used by adapters to create a new connection.
   """
@@ -22,7 +36,6 @@ defmodule Plug.Conn.Adapter do
       adapter: adapter,
       host: host,
       method: method,
-      owner: self(),
       path_info: split_path(path),
       port: port,
       remote_ip: remote_ip,
diff --git a/test/plug/conn_test.exs b/test/plug/conn_test.exs
index 371021ed..8b2ef7ed 100644
--- a/test/plug/conn_test.exs
+++ b/test/plug/conn_test.exs
@@ -1440,8 +1440,7 @@ defmodule Plug.ConnTest do
     end
 
     conn = %Conn{
-      adapter: {RaisesOnEmptyChunkAdapter, %{chunks: ""}},
-      owner: self(),
+      adapter: {RaisesOnEmptyChunkAdapter, %{chunks: "", owner: self()}},
       state: :unset
     }