diff --git a/app/validators/unique_username_validator.rb b/app/validators/unique_username_validator.rb
index fb67105dd..4e24e3f5f 100644
--- a/app/validators/unique_username_validator.rb
+++ b/app/validators/unique_username_validator.rb
@@ -1,10 +1,12 @@
 # frozen_string_literal: true
 
+# See also: USERNAME_RE in the Account class
+
 class UniqueUsernameValidator < ActiveModel::Validator
   def validate(account)
     return if account.username.nil?
 
-    normalized_username = account.username.downcase.delete('.')
+    normalized_username = account.username.downcase
 
     scope = Account.where(domain: nil).where('lower(username) = ?', normalized_username)
     scope = scope.where.not(id: account.id) if account.persisted?
diff --git a/spec/models/account_spec.rb b/spec/models/account_spec.rb
index 6495a6193..3a17d540a 100644
--- a/spec/models/account_spec.rb
+++ b/spec/models/account_spec.rb
@@ -583,12 +583,29 @@ RSpec.describe Account, type: :model do
         expect(account.valid?).to be true
       end
 
+      it 'is valid if we are creating an instance actor account with a period' do
+        account = Fabricate.build(:account, id: -99, actor_type: 'Application', locked: true, username: 'example.com')
+        expect(account.valid?).to be true
+      end
+
+      it 'is valid if we are creating a possibly-conflicting instance actor account' do
+        account_1 = Fabricate(:account, username: 'examplecom')
+        account_2 = Fabricate.build(:account, id: -99, actor_type: 'Application', locked: true, username: 'example.com')
+        expect(account_2.valid?).to be true
+      end
+
       it 'is invalid if the username doesn\'t only contains letters, numbers and underscores' do
         account = Fabricate.build(:account, username: 'the-doctor')
         account.valid?
         expect(account).to model_have_error_on_field(:username)
       end
 
+      it 'is invalid if the username contains a period' do
+        account = Fabricate.build(:account, username: 'the.doctor')
+        account.valid?
+        expect(account).to model_have_error_on_field(:username)
+      end
+
       it 'is invalid if the username is longer then 30 characters' do
         account = Fabricate.build(:account, username: Faker::Lorem.characters(31))
         account.valid?