diff --git a/gems/aws-sdk-core/CHANGELOG.md b/gems/aws-sdk-core/CHANGELOG.md
index 82d5d71ba75..2ab142725be 100644
--- a/gems/aws-sdk-core/CHANGELOG.md
+++ b/gems/aws-sdk-core/CHANGELOG.md
@@ -1,6 +1,8 @@
 Unreleased Changes
 ------------------
 
+* Issue - Preserve existing Content-Encoding when applying request trailer checksum.
+
 3.241.1 (2026-01-06)
 ------------------
 
diff --git a/gems/aws-sdk-core/lib/aws-sdk-core/plugins/checksum_algorithm.rb b/gems/aws-sdk-core/lib/aws-sdk-core/plugins/checksum_algorithm.rb
index 31e2010b546..c172ef678d0 100644
--- a/gems/aws-sdk-core/lib/aws-sdk-core/plugins/checksum_algorithm.rb
+++ b/gems/aws-sdk-core/lib/aws-sdk-core/plugins/checksum_algorithm.rb
@@ -381,7 +381,12 @@ def apply_request_trailer_checksum(context, headers, checksum_properties)
           location_name = checksum_properties[:name]
 
           # set required headers
-          headers['Content-Encoding'] = 'aws-chunked'
+          headers['Content-Encoding'] =
+            if headers['Content-Encoding']
+              headers['Content-Encoding'] += ', aws-chunked'
+            else
+              'aws-chunked'
+            end
           headers['X-Amz-Content-Sha256'] = 'STREAMING-UNSIGNED-PAYLOAD-TRAILER'
           headers['X-Amz-Trailer'] = location_name
 
diff --git a/gems/aws-sdk-s3/spec/plugins/checksum_algorithm_spec.rb b/gems/aws-sdk-s3/spec/plugins/checksum_algorithm_spec.rb
index 3590e860294..d3452c6e462 100644
--- a/gems/aws-sdk-s3/spec/plugins/checksum_algorithm_spec.rb
+++ b/gems/aws-sdk-s3/spec/plugins/checksum_algorithm_spec.rb
@@ -111,6 +111,18 @@ module Plugins
         end
       end
 
+      context 'request trailer checksum', skip: defined?(JRUBY_VERSION) do
+        it 'sets aws-chunked when no existing Content-Encoding header' do
+          resp = client.put_object(bucket: bucket, key: key, body: body)
+          expect(resp.context.http_request.headers['Content-Encoding']).to eq('aws-chunked')
+        end
+
+        it 'appends aws-chunked to existing Content-Encoding header' do
+          resp = client.put_object(bucket: bucket, key: key, body: body, content_encoding: 'gzip')
+          expect(resp.context.http_request.headers['Content-Encoding']).to eq('gzip, aws-chunked')
+        end
+      end
+
       context 'AwsChunkedTrailerDigestIO' do
         let(:subject) do
           ChecksumAlgorithm::AwsChunkedTrailerDigestIO.new(