diff --git a/History.md b/History.md
index 93bd8467..83372dd8 100644
--- a/History.md
+++ b/History.md
@@ -2,6 +2,7 @@
 
 * Bugfixes
   * Add `#flush` and `#sync` methods to `Puma::NullIO`  ([#2553])
+  * Restore `sync=true` on `STDOUT` and `STDERR` streams #2557
 
 ## 5.2.1 / 2021-02-05
 
diff --git a/lib/puma/configuration.rb b/lib/puma/configuration.rb
index 3f0990bd..d7c18ad1 100644
--- a/lib/puma/configuration.rb
+++ b/lib/puma/configuration.rb
@@ -206,7 +206,8 @@ module Puma
         :first_data_timeout => Const::FIRST_DATA_TIMEOUT,
         :raise_exception_on_sigterm => true,
         :max_fast_inline => Const::MAX_FAST_INLINE,
-        :io_selector_backend => :auto
+        :io_selector_backend => :auto,
+        :mutate_stdout_and_stderr_to_sync_on_write => true,
       }
     end
 
diff --git a/lib/puma/dsl.rb b/lib/puma/dsl.rb
index ce73f16d..45511e07 100644
--- a/lib/puma/dsl.rb
+++ b/lib/puma/dsl.rb
@@ -888,5 +888,9 @@ module Puma
     def io_selector_backend(backend)
       @options[:io_selector_backend] = backend.to_sym
     end
+
+    def mutate_stdout_and_stderr_to_sync_on_write(enabled=true)
+      @options[:mutate_stdout_and_stderr_to_sync_on_write] = enabled
+    end
   end
 end
diff --git a/lib/puma/runner.rb b/lib/puma/runner.rb
index c4379bf8..30c4a89b 100644
--- a/lib/puma/runner.rb
+++ b/lib/puma/runner.rb
@@ -126,6 +126,11 @@ module Puma
         STDERR.puts "=== puma startup: #{Time.now} ==="
         STDERR.flush unless STDERR.sync
       end
+
+      if @options[:mutate_stdout_and_stderr_to_sync_on_write]
+        STDOUT.sync = true
+        STDERR.sync = true
+      end
     end
 
     def load_and_bind
diff --git a/test/rackup/write_to_stdout.ru b/test/rackup/write_to_stdout.ru
new file mode 100644
index 00000000..880d3d2e
--- /dev/null
+++ b/test/rackup/write_to_stdout.ru
@@ -0,0 +1,6 @@
+app = lambda do |env|
+  $stdout.write "hello\n"
+  [200, {"Content-Type" => "text/plain"}, ["Hello World"]]
+end
+
+run app
diff --git a/test/test_integration_single.rb b/test/test_integration_single.rb
index 2d2ce3a2..1d39fb04 100644
--- a/test/test_integration_single.rb
+++ b/test/test_integration_single.rb
@@ -163,4 +163,11 @@ class TestIntegrationSingle < TestIntegration
     assert(!File.file?("t2-pid"))
     assert_equal("Puma is started\n", out)
   end
+
+  def test_application_logs_are_flushed_on_write
+    cli_server 'test/rackup/write_to_stdout.ru'
+    read_body connect
+    log_line = @server.gets
+    assert_equal "hello\n", log_line
+  end
 end