diff --git a/lib/phoenix/endpoint/cowboy2_adapter.ex b/lib/phoenix/endpoint/cowboy2_adapter.ex
index 84117a9c7a..13abcdcca4 100644
--- a/lib/phoenix/endpoint/cowboy2_adapter.ex
+++ b/lib/phoenix/endpoint/cowboy2_adapter.ex
@@ -80,7 +80,7 @@ defmodule Phoenix.Endpoint.Cowboy2Adapter do
       Application.ensure_all_started(:ssl)
     end
 
-    ref = Module.concat(endpoint, scheme |> Atom.to_string() |> String.upcase())
+    ref = build_ref(endpoint, scheme)
 
     plug =
       if code_reloader? do
@@ -133,6 +133,18 @@ defmodule Phoenix.Endpoint.Cowboy2Adapter do
     _ -> scheme
   end
 
+  @doc false
+  def dynamic_port(endpoint, scheme) do
+    case :ranch.get_addr(build_ref(endpoint, scheme)) do
+      {:local, _unix_path} -> raise "Trying to determine dynamic port, but the adapter is configured to use unix sockets. Remove the configuration for `[port: 0]` on the #{scheme} scheme."
+      {_addr, port} -> port
+    end
+  end
+
+  defp build_ref(endpoint, scheme) do
+    Module.concat(endpoint, scheme |> Atom.to_string() |> String.upcase())
+  end
+
   # TODO: Remove this once {:system, env_var} deprecation is removed
   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)
diff --git a/lib/phoenix/endpoint/supervisor.ex b/lib/phoenix/endpoint/supervisor.ex
index d1e66a113f..65666f32cf 100644
--- a/lib/phoenix/endpoint/supervisor.ex
+++ b/lib/phoenix/endpoint/supervisor.ex
@@ -351,8 +351,8 @@ defmodule Phoenix.Endpoint.Supervisor do
 
     {scheme, port} =
       cond do
-        https -> {"https", https[:port] || 443}
-        http -> {"http", http[:port] || 80}
+        https -> {"https", transport_port(endpoint, https, :https)}
+        http -> {"http", transport_port(endpoint, http, :http)}
         true -> {"http", 80}
       end
 
@@ -369,6 +369,39 @@ defmodule Phoenix.Endpoint.Supervisor do
     %URI{scheme: scheme, port: port, host: host}
   end
 
+  defp transport_port(endpoint, config, scheme) do
+    cond do
+      value = config[:port] ->
+        value
+        |> port_to_integer()
+        |> handle_dynamic_port(endpoint, scheme)
+
+      scheme == :https ->
+        443
+
+      scheme == :http ->
+        80
+    end
+  end
+
+  def handle_dynamic_port(0, endpoint, scheme) do
+    config =
+      cond do
+        server = endpoint.config(:server) -> [server: server]
+        true -> []
+      end
+
+    adapter = endpoint.config(:adapter) || Phoenix.Endpoint.Cowboy2Adapter
+
+    if function_exported?(adapter, :dynamic_port, 2) and server?(config) do
+      adapter.dynamic_port(endpoint, scheme)
+    else
+      0
+    end
+  end
+
+  def handle_dynamic_port(port, _, _), do: port
+
   defp warmup_static(endpoint, %{"latest" => latest, "digests" => digests}) do
     Phoenix.Config.put_new(endpoint, :cache_static_manifest_latest, latest)
     with_vsn? = !endpoint.config(:cache_manifest_skip_vsn)
diff --git a/test/phoenix/endpoint/endpoint_test.exs b/test/phoenix/endpoint/endpoint_test.exs
index beb946a994..a72ddd375a 100644
--- a/test/phoenix/endpoint/endpoint_test.exs
+++ b/test/phoenix/endpoint/endpoint_test.exs
@@ -205,6 +205,28 @@ defmodule Phoenix.Endpoint.EndpointTest do
     assert_receive {^pid, :sample}
   end
 
+  @tag :capture_log
+  test "uses bound dynamic port if server is started" do
+    Application.put_env(:phoenix, __MODULE__.DynamicPortStartedEndpoint, http: [port: 0], server: true)
+    defmodule DynamicPortStartedEndpoint do
+      use Phoenix.Endpoint, otp_app: :phoenix
+    end
+    DynamicPortStartedEndpoint.start_link()
+    assert %{port: port} = DynamicPortStartedEndpoint.url() |> URI.parse()
+    assert port > 1023
+  end
+
+  @tag :capture_log
+  test "uses dynamic port 0 if server is not started" do
+    Application.put_env(:phoenix, __MODULE__.DynamicPortEndpoint, http: [port: 0], server: false)
+    defmodule DynamicPortEndpoint do
+      use Phoenix.Endpoint, otp_app: :phoenix
+    end
+    DynamicPortEndpoint.start_link()
+    assert %{port: port} = DynamicPortEndpoint.url() |> URI.parse()
+    assert port == 0
+  end
+
   @tag :capture_log
   test "uses url configuration for static path" do
     Application.put_env(:phoenix, __MODULE__.UrlEndpoint, url: [path: "/api"])