diff --git a/lib/plug/conn.ex b/lib/plug/conn.ex
index 92ea6653..2d73ce33 100644
--- a/lib/plug/conn.ex
+++ b/lib/plug/conn.ex
@@ -658,11 +658,12 @@ defmodule Plug.Conn do
     conn
   end
 
-  def merge_resp_headers(%Conn{resp_headers: current} = conn, headers) do
+  def merge_resp_headers(%Conn{resp_headers: current, adapter: adapter} = conn, headers) do
     headers =
-      Enum.reduce headers, current, fn
-        {key, value}, acc when is_binary(key) and is_binary(value) ->
-          List.keystore(acc, key, 0, {key, value})
+      Enum.reduce headers, current, fn {key, value}, acc when is_binary(key) and is_binary(value) ->
+        validate_header_key_if_test!(adapter, key)
+        validate_header_value!(key, value)
+        List.keystore(acc, key, 0, {key, value})
       end
     %{conn | resp_headers: headers}
   end
diff --git a/test/plug/conn_test.exs b/test/plug/conn_test.exs
index 961854e4..47ce0af9 100644
--- a/test/plug/conn_test.exs
+++ b/test/plug/conn_test.exs
@@ -409,6 +409,16 @@ defmodule Plug.ConnTest do
     end
   end
 
+  test "merge_resp_header/3 raises when invalid header value given" do
+    assert_raise Plug.Conn.InvalidHeaderError, ~S[value for header "x-sample" contains control feed (\r) or newline (\n): "value\rBAR"], fn ->
+      merge_resp_headers(conn(:get, "foo"), [{"x-sample", "value\rBAR"}])
+    end
+
+    assert_raise Plug.Conn.InvalidHeaderError, ~S[value for header "x-sample" contains control feed (\r) or newline (\n): "value\n\nBAR"], fn ->
+      merge_resp_headers(conn(:get, "foo"), [{"x-sample", "value\n\nBAR"}])
+    end
+  end
+
   test "merge_resp_headers/3" do
     conn1 = merge_resp_headers(conn(:head, "/foo"), %{"x-foo" => "bar", "x-bar" => "baz"})
     assert get_resp_header(conn1, "x-foo") == ["bar"]