diff --git a/app/validators/qualified_domain_array_validator.rb b/app/validators/qualified_domain_array_validator.rb
new file mode 100644
index 00000000000..986c146a9db
--- /dev/null
+++ b/app/validators/qualified_domain_array_validator.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+# QualifiedDomainArrayValidator
+#
+# Custom validator for URL hosts/'qualified domains' (FQDNs, ex: gitlab.com, sub.example.com).
+# This does not check if the domain actually exists. It only checks if it is a
+# valid domain string.
+#
+# Example:
+#
+#   class ApplicationSetting < ApplicationRecord
+#     validates :outbound_local_requests_whitelist, qualified_domain_array: true
+#   end
+#
+class QualifiedDomainArrayValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    validate_value_present(record, attribute, value)
+    validate_host_length(record, attribute, value)
+    validate_idna_encoding(record, attribute, value)
+    validate_sanitization(record, attribute, value)
+  end
+
+  private
+
+  def validate_value_present(record, attribute, value)
+    return unless value.blank?
+
+    record.errors.add(attribute, _('entries cannot be blank'))
+  end
+
+  def validate_host_length(record, attribute, value)
+    return unless value&.any? { |entry| entry.size > 255 }
+
+    record.errors.add(attribute, _('entries cannot be larger than 255 characters'))
+  end
+
+  def validate_idna_encoding(record, attribute, value)
+    return if value&.all?(&:ascii_only?)
+
+    record.errors.add(attribute, _('unicode domains should use IDNA encoding'))
+  end
+
+  def validate_sanitization(record, attribute, value)
+    sanitizer = Rails::Html::FullSanitizer.new
+    return unless value&.any? { |str| sanitizer.sanitize(str) != str }
+
+    record.errors.add(attribute, _('entries cannot contain HTML tags'))
+  end
+end
diff --git a/locale/gitlab.pot b/locale/gitlab.pot
index bd26ca6714d..5cd9aa7e2de 100644
--- a/locale/gitlab.pot
+++ b/locale/gitlab.pot
@@ -12803,6 +12803,15 @@ msgstr ""
 msgid "encrypted: needs to be a :required, :optional or :migrating!"
 msgstr ""
 
+msgid "entries cannot be blank"
+msgstr ""
+
+msgid "entries cannot be larger than 255 characters"
+msgstr ""
+
+msgid "entries cannot contain HTML tags"
+msgstr ""
+
 msgid "error"
 msgstr ""
 
@@ -13308,6 +13317,9 @@ msgstr ""
 msgid "triggered"
 msgstr ""
 
+msgid "unicode domains should use IDNA encoding"
+msgstr ""
+
 msgid "updated"
 msgstr ""
 
diff --git a/spec/validators/qualified_domain_array_validator_spec.rb b/spec/validators/qualified_domain_array_validator_spec.rb
new file mode 100644
index 00000000000..a96b00bfd1d
--- /dev/null
+++ b/spec/validators/qualified_domain_array_validator_spec.rb
@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe QualifiedDomainArrayValidator do
+  class TestClass
+    include ActiveModel::Validations
+
+    attr_accessor :domain_array
+
+    def initialize(domain_array)
+      self.domain_array = domain_array
+    end
+  end
+
+  let!(:record) do
+    TestClass.new(['gitlab.com'])
+  end
+
+  subject { validator.validate(record) }
+
+  shared_examples 'cannot be blank' do
+    it 'returns error when attribute is blank' do
+      record.domain_array = []
+
+      subject
+
+      expect(record.errors).to be_present
+      expect(record.errors.first[1]).to eq 'entries cannot be blank'
+    end
+  end
+
+  shared_examples 'can be nil' do
+    it 'allows when attribute is nil' do
+      record.domain_array = nil
+
+      subject
+
+      expect(record.errors).to be_empty
+    end
+  end
+
+  describe 'validations' do
+    let(:validator) { described_class.new(attributes: [:domain_array]) }
+
+    it_behaves_like 'cannot be blank'
+
+    it 'returns error when attribute is nil' do
+      record.domain_array = nil
+
+      subject
+
+      expect(record.errors).to be_present
+    end
+
+    it 'allows when domain is valid' do
+      subject
+
+      expect(record.errors).to be_empty
+    end
+
+    it 'returns error when domain contains unicode' do
+      record.domain_array = ['ğitlab.com']
+
+      subject
+
+      expect(record.errors).to be_present
+      expect(record.errors.first[1]).to eq 'unicode domains should use IDNA encoding'
+    end
+
+    it 'returns error when entry is larger than 255 chars' do
+      record.domain_array = ['a' * 256]
+
+      subject
+
+      expect(record.errors).to be_present
+      expect(record.errors.first[1]).to eq 'entries cannot be larger than 255 characters'
+    end
+
+    it 'returns error when entry contains HTML tags' do
+      record.domain_array = ['gitlab.com<h1>something</h1>']
+
+      subject
+
+      expect(record.errors).to be_present
+      expect(record.errors.first[1]).to eq 'entries cannot contain HTML tags'
+    end
+  end
+
+  context 'when allow_nil is set to true' do
+    let(:validator) { described_class.new(attributes: [:domain_array], allow_nil: true) }
+
+    it_behaves_like 'can be nil'
+
+    it_behaves_like 'cannot be blank'
+  end
+
+  context 'when allow_blank is set to true' do
+    let(:validator) { described_class.new(attributes: [:domain_array], allow_blank: true) }
+
+    it_behaves_like 'can be nil'
+
+    it 'allows when attribute is blank' do
+      record.domain_array = []
+
+      subject
+
+      expect(record.errors).to be_empty
+    end
+  end
+end