diff --git a/Library/Homebrew/cmd/untap.rb b/Library/Homebrew/cmd/untap.rb
index b83667fed6..66529cc027 100644
--- a/Library/Homebrew/cmd/untap.rb
+++ b/Library/Homebrew/cmd/untap.rb
@@ -28,6 +28,24 @@ module Homebrew
       tap = Tap.fetch(tapname)
       odie "Untapping #{tap} is not allowed" if tap.core_tap?
 
+      installed_tap_formulae = Formula.installed.select { |formula| formula.tap == tap }
+      installed_tap_casks = Cask::Caskroom.casks.select { |cask| cask.tap == tap }
+
+      if installed_tap_formulae.present? || installed_tap_casks.present?
+        installed_names = (installed_tap_formulae + installed_tap_casks.map(&:token)).join("\n")
+        if args.force? || Homebrew::EnvConfig.developer?
+          opoo <<~EOS
+            Untapping #{tap} even though it contains the following installed formulae or casks:
+            #{installed_names}
+          EOS
+        else
+          odie <<~EOS
+            Refusing to untap #{tap} because it contains the following installed formulae or casks:
+            #{installed_names}
+          EOS
+        end
+      end
+
       tap.uninstall
     end
   end