diff --git a/Changes.md b/Changes.md
index 9ebdf5498..530b0a140 100644
--- a/Changes.md
+++ b/Changes.md
@@ -5,6 +5,7 @@
 Unreleased
 ---------
 
+- Ensure `Rack::ContentLength` is loaded as middleware for correct Web UI responses [#4541]
 - Avoid exception dumping SSL store in Redis connection logging [#4532]
 
 6.0.7
diff --git a/lib/sidekiq/web.rb b/lib/sidekiq/web.rb
index 1fc0c47e3..3422bc510 100644
--- a/lib/sidekiq/web.rb
+++ b/lib/sidekiq/web.rb
@@ -12,6 +12,7 @@
 require "sidekiq/web/application"
 
 require "rack/protection"
+require "rack/content_length"
 
 require "rack/builder"
 require "rack/file"
@@ -172,6 +173,13 @@ def build_sessions
 
         middlewares.unshift [[::Rack::Session::Cookie, options], nil]
       end
+
+      # Since Sidekiq::WebApplication no longer calculates its own
+      # Content-Length response header, we must ensure that the Rack middleware
+      # that does this is loaded
+      unless using? ::Rack::ContentLength
+        middlewares.unshift [[::Rack::ContentLength], nil]
+      end
     end
 
     def build