diff --git a/app/logical/user_name_validator.rb b/app/logical/user_name_validator.rb
index 8d44cb064..99ecce9dc 100644
--- a/app/logical/user_name_validator.rb
+++ b/app/logical/user_name_validator.rb
@@ -7,14 +7,33 @@
 #
 # @see https://guides.rubyonrails.org/active_record_validations.html#custom-validators
 class UserNameValidator < ActiveModel::EachValidator
-  def validate_each(rec, attr, value)
-    name = value
+  ALLOWED_PUNCTUATION = "_.-" # All other punctuation characters are forbidden
 
-    rec.errors.add(attr, "already exists") if User.find_by_name(name).present?
-    rec.errors.add(attr, "must be more than 1 character long") if name.length <= 1
-    rec.errors.add(attr, "must be less than 25 characters long") if name.length >= 25
-    rec.errors.add(attr, "cannot have whitespace or colons") if name =~ /[[:space:]]|:/
-    rec.errors.add(attr, "cannot begin or end with an underscore") if name =~ /\A_|_\z/
-    rec.errors.add(attr, "is not allowed") if name =~ Regexp.union(Danbooru.config.user_name_blacklist)
+  def validate_each(rec, attr, name)
+    forbidden_characters = name.delete(ALLOWED_PUNCTUATION).chars.grep(/[[:punct:]]/).uniq
+
+    if rec.new_record? && User.find_by_name(name).present?
+      rec.errors.add(attr, "already exists")
+    elsif name.length <= 1
+      rec.errors.add(attr, "must be more than 1 character long")
+    elsif name.length >= 25
+      rec.errors.add(attr, "must be less than 25 characters long")
+    elsif name =~ /[[:space:]]/
+      rec.errors.add(attr, "can't contain whitespace")
+    elsif name =~ /\A[[:punct:]]/
+      rec.errors.add(attr, "can't start with '#{name.first}'")
+    elsif name =~ /[[:punct:]]\z/
+      rec.errors.add(attr, "can't end with '#{name.last}'")
+    elsif name =~ /__/
+      rec.errors.add(attr, "can't contain multiple underscores in a row")
+    elsif forbidden_characters.present?
+      rec.errors.add(attr, "can't contain #{forbidden_characters.map { |c| "'#{c}'" }.to_sentence}")
+    elsif name !~ /\A([a-zA-Z0-9]|\p{Han}|\p{Hangul}|\p{Hiragana}|\p{Katakana}|[#{ALLOWED_PUNCTUATION}])+\z/
+      rec.errors.add(attr, "must contain only basic letters or numbers")
+    elsif name =~ /\Auser_\d+\z/i
+      rec.errors.add(attr, "can't be the same as a deleted user")
+    elsif name =~ Regexp.union(Danbooru.config.user_name_blacklist)
+      rec.errors.add(attr, "is not allowed")
+    end
   end
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index 7a55daa9d..323fe94d2 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class User < ApplicationRecord
+  extend Memoist
+
   class PrivilegeError < StandardError; end
 
   module Levels
@@ -204,6 +206,18 @@ class User < ApplicationRecord
         errors.add(:base, "Can't enable privacy mode without a Gold account")
       end
     end
+
+    def name_errors
+      User.validators_on(:name).each do |validator|
+        validator.validate_each(self, :name, name)
+      end
+
+      errors
+    end
+
+    def name_invalid?
+      name_errors.present?
+    end
   end
 
   concerning :AuthenticationMethods do
@@ -695,4 +709,6 @@ class User < ApplicationRecord
   def self.available_includes
     [:inviter]
   end
+
+  memoize :name_errors
 end
diff --git a/app/views/layouts/default.html.erb b/app/views/layouts/default.html.erb
index fe035f1fb..5c45e848c 100644
--- a/app/views/layouts/default.html.erb
+++ b/app/views/layouts/default.html.erb
@@ -104,6 +104,13 @@
       <%= render "users/dmail_notice" %>
     <% end %>
 
+    <% if !CurrentUser.user.is_anonymous? && CurrentUser.user.name_invalid? %>
+      <div class="notice notice-error notice-large" id="invalid-name-notice">
+        <h2>Action required </h2>
+        <div>You must <%= link_to "change your username", new_user_name_change_request_path %> to continue using <%= Danbooru.config.canonical_app_name %>.</div>
+      </div>
+    <% end %>
+
     <div class="notice notice-info" id="notice" style="<%= "display: none;" unless flash[:notice] %>">
       <span class="prose"><%= format_text(flash[:notice], inline: true) %>.</span>
       <a href="#" id="close-notice-link">close</a>
diff --git a/app/views/user_name_change_requests/new.html.erb b/app/views/user_name_change_requests/new.html.erb
index b1be8501d..0c7698c55 100644
--- a/app/views/user_name_change_requests/new.html.erb
+++ b/app/views/user_name_change_requests/new.html.erb
@@ -1,13 +1,32 @@
 <div id="c-user-name-change-requests">
-  <div id="a-new">
-    <h1>Name Change Request</h1>
+  <div id="a-new" class="fixed-width-container">
+    <h1>Change Name</h1>
 
-    <p>You can request a name change once per week. Your previous names will still
-    be visible on your profile to other Danbooru members, but they won't be visible
-    to search engines.</p>
+    <% if CurrentUser.user.name_invalid? %>
+      <p> Your current username is invalid. You must change your username to continue
+      using <%= Danbooru.config.canonical_app_name %>.</p>
+
+      <p>
+        Current name: <b><%= CurrentUser.user.name %></b>.<br>
+        Error: <%= CurrentUser.user.name_errors.full_messages.join(". ").html_safe %>.
+      </p>
+    <% end %>
+
+    <div class="prose mt-4 mb-4">
+      <h6>Rules</h6>
+      <ul>
+        <li>Names can contain only letters, numbers, underscore ('_'), period ('.'), and dash ('-') characters.</li>
+        <li>Names must start and end with a letter or number.</li>
+        <li>Names must be less than 25 characters long.</li>
+        <li>Names can't insult or impersonate other users.</li>
+        <li>Names are case-insensitive.</li>
+        <li>Your previous names will be visible on your profile to other Danbooru members, but they won't be visible to search engines.</li>
+        <li>You can't change your name more than once per week.</li>
+      </ul>
+    </div>
 
     <%= edit_form_for(@change_request) do |f| %>
-      <%= f.input :desired_name, label: "Name" %>
+      <%= f.input :desired_name, label: "New name" %>
       <%= f.input :desired_name_confirmation, label: "Confirm name" %>
       <%= f.submit "Submit", "data-confirm": "Are you sure you want to change your name?" %>
     <% end %>
diff --git a/test/functional/application_controller_test.rb b/test/functional/application_controller_test.rb
index 53ff6a86e..8fa07e359 100644
--- a/test/functional/application_controller_test.rb
+++ b/test/functional/application_controller_test.rb
@@ -75,6 +75,17 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
       end
     end
 
+    context "when a user has an invalid username" do
+      should "show a warning banner" do
+        @user = create(:user)
+        @user.update_columns(name: "foo__bar")
+
+        get_auth posts_path, @user
+        assert_response :success
+        assert_select "#invalid-name-notice"
+      end
+    end
+
     context "on api authentication" do
       setup do
         @user = create(:user, password: "password")
diff --git a/test/functional/user_name_change_requests_controller_test.rb b/test/functional/user_name_change_requests_controller_test.rb
index b9276d24b..d8e6abf40 100644
--- a/test/functional/user_name_change_requests_controller_test.rb
+++ b/test/functional/user_name_change_requests_controller_test.rb
@@ -12,6 +12,13 @@ class UserNameChangeRequestsControllerTest < ActionDispatch::IntegrationTest
         get_auth new_user_name_change_request_path, @user
         assert_response :success
       end
+
+      should "render when the current user's name is invalid" do
+        @user.update_columns(name: "foo__bar")
+        get_auth new_user_name_change_request_path, @user
+
+        assert_response :success
+      end
     end
 
     context "create action" do
diff --git a/test/unit/user_test.rb b/test/unit/user_test.rb
index 1006bc6cb..3bcbf157c 100644
--- a/test/unit/user_test.rb
+++ b/test/unit/user_test.rb
@@ -148,27 +148,51 @@ class UserTest < ActiveSupport::TestCase
     context "name" do
       should "not contain whitespace" do
         # U+2007: https://en.wikipedia.org/wiki/Figure_space
-        user = FactoryBot.build(:user, :name => "foo\u2007bar")
+        user = build(:user, name: "foo\u2007bar")
         user.save
-        assert_equal(["Name cannot have whitespace or colons"], user.errors.full_messages)
+        assert_equal(["Name can't contain whitespace"], user.errors.full_messages)
+      end
+
+      should "be less than 25 characters long" do
+        user = build(:user, name: "a"*25)
+        user.save
+        assert_equal(["Name must be less than 25 characters long"], user.errors.full_messages)
       end
 
       should "not contain a colon" do
-        user = FactoryBot.build(:user, :name => "a:b")
+        user = build(:user, name: "a:b")
         user.save
-        assert_equal(["Name cannot have whitespace or colons"], user.errors.full_messages)
+        assert_equal(["Name can't contain ':'"], user.errors.full_messages)
       end
 
       should "not begin with an underscore" do
-        user = FactoryBot.build(:user, :name => "_x")
+        user = build(:user, name: "_x")
         user.save
-        assert_equal(["Name cannot begin or end with an underscore"], user.errors.full_messages)
+        assert_equal(["Name can't start with '_'"], user.errors.full_messages)
       end
 
       should "not end with an underscore" do
-        user = FactoryBot.build(:user, :name => "x_")
+        user = build(:user, name: "x_")
         user.save
-        assert_equal(["Name cannot begin or end with an underscore"], user.errors.full_messages)
+        assert_equal(["Name can't end with '_'"], user.errors.full_messages)
+      end
+
+      should "not contain consecutive underscores" do
+        user = build(:user, name: "x__y")
+        user.save
+        assert_equal(["Name can't contain multiple underscores in a row"], user.errors.full_messages)
+      end
+
+      should "not allow non-ASCII characters" do
+        user = build(:user, name: "Pokémon")
+        user.save
+        assert_equal(["Name must contain only basic letters or numbers"], user.errors.full_messages)
+      end
+
+      should "not be in the same format as a deleted user" do
+        user = build(:user, name: "user_1234")
+        user.save
+        assert_equal(["Name can't be the same as a deleted user"], user.errors.full_messages)
       end
 
       should "not allow blacklisted names" do
@@ -177,11 +201,6 @@ class UserTest < ActiveSupport::TestCase
         user.save
         assert_equal(["Name is not allowed"], user.errors.full_messages)
       end
-
-      should "be updated" do
-        @user = FactoryBot.create(:user)
-        @user.update_attribute(:name, "danzig")
-      end
     end
 
     context "searching for users by name" do
@@ -206,10 +225,14 @@ class UserTest < ActiveSupport::TestCase
         assert_nil(User.name_to_id("does_not_exist"))
       end
 
-      should "work for names containing asterisks or backlashes" do
-        @user1 = create(:user, name: "user*1")
-        @user2 = create(:user, name: "user*2")
-        @user3 = create(:user, name: "user\*3")
+      should "work for names containing asterisks or backslashes" do
+        @user1 = build(:user, name: "user*1")
+        @user2 = build(:user, name: "user*2")
+        @user3 = build(:user, name: "user\*3")
+
+        @user1.save(validate: false)
+        @user2.save(validate: false)
+        @user3.save(validate: false)
 
         assert_equal(@user1.id, User.find_by_name("user*1").id)
         assert_equal(@user2.id, User.find_by_name("user*2").id)
@@ -259,9 +282,13 @@ class UserTest < ActiveSupport::TestCase
 
     context "when searched by name" do
       should "match wildcards" do
-        user1 = FactoryBot.create(:user, :name => "foo")
-        user2 = FactoryBot.create(:user, :name => "foo*bar")
-        user3 = FactoryBot.create(:user, :name => "bar\*baz")
+        user1 = build(:user, name: "foo")
+        user2 = build(:user, name: "foo*bar")
+        user3 = build(:user, name: "bar\*baz")
+
+        user1.save(validate: false)
+        user2.save(validate: false)
+        user3.save(validate: false)
 
         assert_equal([user2.id, user1.id], User.search(name: "foo*").map(&:id))
         assert_equal([user2.id], User.search(name: "foo\*bar").map(&:id))