diff --git a/CHANGELOG.md b/CHANGELOG.md
index c4f3867a2..905e35f26 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -49,6 +49,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Admin API: An endpoint to manage frontends.
 - Streaming API: Add follow relationships updates.
 - WebPush: Introduce `pleroma:chat_mention` and `pleroma:emoji_reaction` notification types
+- Mastodon API: Add monthly active users to `/api/v1/instance` (`pleroma.stats.mau`)
 </details>
 
 ### Fixed
diff --git a/lib/pleroma/user.ex b/lib/pleroma/user.ex
index e422b59f1..06cdb42af 100644
--- a/lib/pleroma/user.ex
+++ b/lib/pleroma/user.ex
@@ -146,6 +146,7 @@ defmodule Pleroma.User do
     field(:inbox, :string)
     field(:shared_inbox, :string)
     field(:accepts_chat_messages, :boolean, default: nil)
+    field(:last_active_at, :naive_datetime)
 
     embeds_one(
       :notification_settings,
@@ -2444,4 +2445,19 @@ defmodule Pleroma.User do
   def get_host(%User{ap_id: ap_id} = _user) do
     URI.parse(ap_id).host
   end
+
+  def update_last_active_at(%__MODULE__{local: true} = user) do
+    user
+    |> cast(%{last_active_at: NaiveDateTime.utc_now()}, [:last_active_at])
+    |> update_and_set_cache()
+  end
+
+  def active_user_count(weeks \\ 4) do
+    active_after = Timex.shift(NaiveDateTime.utc_now(), weeks: -weeks)
+
+    __MODULE__
+    |> where([u], u.last_active_at >= ^active_after)
+    |> where([u], u.local == true)
+    |> Repo.aggregate(:count)
+  end
 end
diff --git a/lib/pleroma/web/mastodon_api/views/instance_view.ex b/lib/pleroma/web/mastodon_api/views/instance_view.ex
index 1edbdbe11..73205fb6d 100644
--- a/lib/pleroma/web/mastodon_api/views/instance_view.ex
+++ b/lib/pleroma/web/mastodon_api/views/instance_view.ex
@@ -45,6 +45,7 @@ defmodule Pleroma.Web.MastodonAPI.InstanceView do
           fields_limits: fields_limits(),
           post_formats: Config.get([:instance, :allowed_post_formats])
         },
+        stats: %{mau: Pleroma.User.active_user_count()},
         vapid_public_key: Keyword.get(Pleroma.Web.Push.vapid_config(), :public_key)
       }
     }
diff --git a/lib/pleroma/web/plugs/user_tracking_plug.ex b/lib/pleroma/web/plugs/user_tracking_plug.ex
new file mode 100644
index 000000000..c9a988f00
--- /dev/null
+++ b/lib/pleroma/web/plugs/user_tracking_plug.ex
@@ -0,0 +1,30 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.UserTrackingPlug do
+  alias Pleroma.User
+
+  import Plug.Conn, only: [assign: 3]
+
+  @update_interval :timer.hours(24)
+
+  def init(opts), do: opts
+
+  def call(%{assigns: %{user: %User{id: id} = user}} = conn, _) when not is_nil(id) do
+    with true <- needs_update?(user),
+         {:ok, user} <- User.update_last_active_at(user) do
+      assign(conn, :user, user)
+    else
+      _ -> conn
+    end
+  end
+
+  def call(conn, _), do: conn
+
+  defp needs_update?(%User{last_active_at: nil}), do: true
+
+  defp needs_update?(%User{last_active_at: last_active_at}) do
+    NaiveDateTime.diff(NaiveDateTime.utc_now(), last_active_at, :millisecond) >= @update_interval
+  end
+end
diff --git a/lib/pleroma/web/router.ex b/lib/pleroma/web/router.ex
index a9e332fa1..7521f5dc3 100644
--- a/lib/pleroma/web/router.ex
+++ b/lib/pleroma/web/router.ex
@@ -56,6 +56,7 @@ defmodule Pleroma.Web.Router do
     plug(Pleroma.Web.Plugs.UserEnabledPlug)
     plug(Pleroma.Web.Plugs.SetUserSessionIdPlug)
     plug(Pleroma.Web.Plugs.EnsureUserTokenAssignsPlug)
+    plug(Pleroma.Web.Plugs.UserTrackingPlug)
   end
 
   pipeline :base_api do
diff --git a/priv/repo/migrations/20210122151424_add_last_active_at_to_users.exs b/priv/repo/migrations/20210122151424_add_last_active_at_to_users.exs
new file mode 100644
index 000000000..9671e495b
--- /dev/null
+++ b/priv/repo/migrations/20210122151424_add_last_active_at_to_users.exs
@@ -0,0 +1,11 @@
+defmodule Pleroma.Repo.Migrations.AddLastActiveAtToUsers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add(:last_active_at, :naive_datetime)
+    end
+
+    create_if_not_exists(index(:users, [:last_active_at]))
+  end
+end
diff --git a/test/pleroma/user_test.exs b/test/pleroma/user_test.exs
index b4df22c2c..ae6fc4c0d 100644
--- a/test/pleroma/user_test.exs
+++ b/test/pleroma/user_test.exs
@@ -2248,4 +2248,43 @@ defmodule Pleroma.UserTest do
     user = insert(:user, ap_id: "https://lain.com/users/lain", nickname: "lain")
     assert User.get_host(user) == "lain.com"
   end
+
+  test "update_last_active_at/1" do
+    user = insert(:user)
+    assert is_nil(user.last_active_at)
+
+    test_started_at = NaiveDateTime.utc_now() |> NaiveDateTime.truncate(:second)
+
+    assert {:ok, user} = User.update_last_active_at(user)
+
+    assert user.last_active_at >= test_started_at
+    assert user.last_active_at <= NaiveDateTime.truncate(NaiveDateTime.utc_now(), :second)
+
+    last_active_at =
+      NaiveDateTime.utc_now()
+      |> NaiveDateTime.add(-:timer.hours(24), :millisecond)
+      |> NaiveDateTime.truncate(:second)
+
+    assert {:ok, user} =
+             user
+             |> cast(%{last_active_at: last_active_at}, [:last_active_at])
+             |> User.update_and_set_cache()
+
+    assert user.last_active_at == last_active_at
+    assert {:ok, user} = User.update_last_active_at(user)
+    assert user.last_active_at >= test_started_at
+    assert user.last_active_at <= NaiveDateTime.truncate(NaiveDateTime.utc_now(), :second)
+  end
+
+  test "active_user_count/1" do
+    insert(:user)
+    insert(:user, %{local: false})
+    insert(:user, %{last_active_at: Timex.shift(NaiveDateTime.utc_now(), weeks: -5)})
+    insert(:user, %{last_active_at: Timex.shift(NaiveDateTime.utc_now(), weeks: -3)})
+    insert(:user, %{last_active_at: NaiveDateTime.utc_now()})
+
+    assert User.active_user_count() == 2
+    assert User.active_user_count(6) == 3
+    assert User.active_user_count(1) == 1
+  end
 end
diff --git a/test/pleroma/web/mastodon_api/controllers/instance_controller_test.exs b/test/pleroma/web/mastodon_api/controllers/instance_controller_test.exs
index 0d4eebb73..b99856659 100644
--- a/test/pleroma/web/mastodon_api/controllers/instance_controller_test.exs
+++ b/test/pleroma/web/mastodon_api/controllers/instance_controller_test.exs
@@ -47,6 +47,7 @@ defmodule Pleroma.Web.MastodonAPI.InstanceControllerTest do
     assert result["pleroma"]["metadata"]["federation"]
     assert result["pleroma"]["metadata"]["fields_limits"]
     assert result["pleroma"]["vapid_public_key"]
+    assert result["pleroma"]["stats"]["mau"] == 0
 
     assert email == from_config_email
     assert thumbnail == from_config_thumbnail
diff --git a/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs b/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs
index a647cd57f..3c73eb514 100644
--- a/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs
+++ b/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs
@@ -263,6 +263,7 @@ defmodule Pleroma.Web.MastodonAPI.StatusControllerTest do
 
       fake_conn =
         conn
+        |> assign(:user, refresh_record(conn.assigns.user))
         |> put_req_header("content-type", "application/json")
         |> post("/api/v1/statuses", %{
           "status" =>
diff --git a/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs b/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs
index 98a23aaaa..54f2c5a58 100644
--- a/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs
+++ b/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs
@@ -104,7 +104,7 @@ defmodule Pleroma.Web.PleromaAPI.ConversationControllerTest do
     [participation] = Participation.for_user(user)
     participation = Repo.preload(participation, :recipients)
 
-    assert user in participation.recipients
+    assert refresh_record(user) in participation.recipients
     assert other_user in participation.recipients
   end
 
diff --git a/test/pleroma/web/plugs/user_tracking_plug_test.exs b/test/pleroma/web/plugs/user_tracking_plug_test.exs
new file mode 100644
index 000000000..8e9d59b99
--- /dev/null
+++ b/test/pleroma/web/plugs/user_tracking_plug_test.exs
@@ -0,0 +1,58 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.UserTrackingPlugTest do
+  use Pleroma.Web.ConnCase, async: true
+
+  import Pleroma.Factory
+
+  alias Pleroma.Web.Plugs.UserTrackingPlug
+
+  test "updates last_active_at for a new user", %{conn: conn} do
+    user = insert(:user)
+
+    assert is_nil(user.last_active_at)
+
+    test_started_at = NaiveDateTime.utc_now() |> NaiveDateTime.truncate(:second)
+
+    %{assigns: %{user: user}} =
+      conn
+      |> assign(:user, user)
+      |> UserTrackingPlug.call(%{})
+
+    assert user.last_active_at >= test_started_at
+    assert user.last_active_at <= NaiveDateTime.truncate(NaiveDateTime.utc_now(), :second)
+  end
+
+  test "doesn't update last_active_at if it was updated recently", %{conn: conn} do
+    last_active_at =
+      NaiveDateTime.utc_now()
+      |> NaiveDateTime.add(-:timer.hours(1), :millisecond)
+      |> NaiveDateTime.truncate(:second)
+
+    user = insert(:user, %{last_active_at: last_active_at})
+
+    %{assigns: %{user: user}} =
+      conn
+      |> assign(:user, user)
+      |> UserTrackingPlug.call(%{})
+
+    assert user.last_active_at == last_active_at
+  end
+
+  test "skips updating last_active_at if user ID is nil", %{conn: conn} do
+    %{assigns: %{user: user}} =
+      conn
+      |> assign(:user, %Pleroma.User{})
+      |> UserTrackingPlug.call(%{})
+
+    assert is_nil(user.last_active_at)
+  end
+
+  test "does nothing if user is not present", %{conn: conn} do
+    %{assigns: assigns} = UserTrackingPlug.call(conn, %{})
+
+    refute Map.has_key?(assigns, :user)
+  end
+end