diff --git a/config/environment.rb b/config/environment.rb
index 52cdf1628..7dc9a1a5e 100644
--- a/config/environment.rb
+++ b/config/environment.rb
@@ -12,5 +12,14 @@ if Dir.glob(File.join(vendor_plugins_dir, "*")).any?
   exit 1
 end
 
+# Make sure there's no themes in public/themes before starting
+public_themes_dir = Rails.public_path.join('themes')
+if Dir.glob(File.join(public_themes_dir, "*")).any?
+  $stderr.puts "Themes in public/themes (#{public_themes_dir}) are no longer allowed. " \
+    "Please, put your Redmine themes in the `themes` directory at the root of your " \
+    "Redmine directory (#{Rails.root.join('themes')})"
+  exit 1
+end
+
 # Initialize the Rails application
 Rails.application.initialize!