diff --git a/config/config.md b/config/config.md
index dbbfa9194..e8e2cf6f8 100644
--- a/config/config.md
+++ b/config/config.md
@@ -63,6 +63,8 @@ This filter replaces the filename (not the path) of an upload. For complete obfu
   * "masto": Copy verbatim, as in Mastodon.
   * "noop": Don't copy the subject.
 * `always_show_subject_input`: When set to false, auto-hide the subject field when it's empty.
+* `extended_nickname_format`: Set to `true` to use extended local nicknames format (allows underscore). This will break federation with
+    older software for theses nicknames.
 
 ## :fe
 This section is used to configure Pleroma-FE, unless ``:managed_config`` in ``:instance`` is set to false.
diff --git a/lib/pleroma/formatter.ex b/lib/pleroma/formatter.ex
index 5b03e9aeb..1fea1c57a 100644
--- a/lib/pleroma/formatter.ex
+++ b/lib/pleroma/formatter.ex
@@ -18,7 +18,7 @@ def parse_tags(text, data \\ %{}) do
   def parse_mentions(text) do
     # Modified from https://www.w3.org/TR/html5/forms.html#valid-e-mail-address
     regex =
-      ~r/@[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]*@?[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*/u
+      ~r/@[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]*@?[a-zA-Z0-9_](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*/u
 
     Regex.scan(regex, text)
     |> List.flatten()
diff --git a/lib/pleroma/user.ex b/lib/pleroma/user.ex
index e146b562c..f10cdfbc8 100644
--- a/lib/pleroma/user.ex
+++ b/lib/pleroma/user.ex
@@ -11,6 +11,11 @@ defmodule Pleroma.User do
 
   @type t :: %__MODULE__{}
 
+  @email_regex ~r/^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/
+
+  @strict_local_nickname_regex ~r/^[a-zA-Z\d]+$/
+  @extended_local_nickname_regex ~r/^[a-zA-Z\d_]+$/
+
   schema "users" do
     field(:bio, :string)
     field(:email, :string)
@@ -77,7 +82,6 @@ def user_info(%User{} = user) do
     }
   end
 
-  @email_regex ~r/^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/
   def remote_user_creation(params) do
     params =
       params
@@ -117,7 +121,7 @@ def update_changeset(struct, params \\ %{}) do
     struct
     |> cast(params, [:bio, :name, :avatar])
     |> unique_constraint(:nickname)
-    |> validate_format(:nickname, ~r/^[a-zA-Z\d]+$/)
+    |> validate_format(:nickname, local_nickname_regex())
     |> validate_length(:bio, max: 5000)
     |> validate_length(:name, min: 1, max: 100)
   end
@@ -134,7 +138,7 @@ def upgrade_changeset(struct, params \\ %{}) do
     struct
     |> cast(params, [:bio, :name, :follower_address, :avatar, :last_refreshed_at])
     |> unique_constraint(:nickname)
-    |> validate_format(:nickname, ~r/^[a-zA-Z\d]+$/)
+    |> validate_format(:nickname, local_nickname_regex())
     |> validate_length(:bio, max: 5000)
     |> validate_length(:name, max: 100)
     |> put_embed(:info, info_cng)
@@ -172,7 +176,7 @@ def register_changeset(struct, params \\ %{}) do
       |> validate_confirmation(:password)
       |> unique_constraint(:email)
       |> unique_constraint(:nickname)
-      |> validate_format(:nickname, ~r/^[a-zA-Z\d]+$/)
+      |> validate_format(:nickname, local_nickname_regex())
       |> validate_format(:email, @email_regex)
       |> validate_length(:bio, max: 1000)
       |> validate_length(:name, min: 1, max: 100)
@@ -861,4 +865,12 @@ defp normalize_tags(tags) do
     |> List.flatten()
     |> Enum.map(&String.downcase(&1))
   end
+
+  defp local_nickname_regex() do
+    if Pleroma.Config.get([:instance, :extended_nickname_format]) do
+      @extended_local_nickname_regex
+    else
+      @strict_local_nickname_regex
+    end
+  end
 end
diff --git a/test/formatter_test.exs b/test/formatter_test.exs
index abb9d882c..428227d78 100644
--- a/test/formatter_test.exs
+++ b/test/formatter_test.exs
@@ -109,13 +109,13 @@ test "turning urls into links" do
 
   describe "add_user_links" do
     test "gives a replacement for user links" do
-      text = "@gsimg According to @archaeme, that is @daggsy. Also hello @archaeme@archae.me"
+      text = "@gsimg According to @archa_eme_, that is @daggsy. Also hello @archaeme@archae.me"
       gsimg = insert(:user, %{nickname: "gsimg"})
 
       archaeme =
         insert(:user, %{
-          nickname: "archaeme",
-          info: %Pleroma.User.Info{source_data: %{"url" => "https://archeme/@archaeme"}}
+          nickname: "archa_eme_",
+          info: %Pleroma.User.Info{source_data: %{"url" => "https://archeme/@archa_eme_"}}
         })
 
       archaeme_remote = insert(:user, %{nickname: "archaeme@archae.me"})
@@ -130,7 +130,7 @@ test "gives a replacement for user links" do
       expected_text =
         "<span><a data-user='#{gsimg.id}' class='mention' href='#{gsimg.ap_id}'>@<span>gsimg</span></a></span> According to <span><a data-user='#{
           archaeme.id
-        }' class='mention' href='#{"https://archeme/@archaeme"}'>@<span>archaeme</span></a></span>, that is @daggsy. Also hello <span><a data-user='#{
+        }' class='mention' href='#{"https://archeme/@archa_eme_"}'>@<span>archa_eme_</span></a></span>, that is @daggsy. Also hello <span><a data-user='#{
           archaeme_remote.id
         }' class='mention' href='#{archaeme_remote.ap_id}'>@<span>archaeme</span></a></span>"