diff --git a/plugins/compress/compress.cc b/plugins/compress/compress.cc
index 58fd52802fb..c06c4c4ec15 100644
--- a/plugins/compress/compress.cc
+++ b/plugins/compress/compress.cc
@@ -772,23 +772,16 @@ transformable(TSHttpTxn txnp, bool server, HostConfiguration *host_configuration
 }
 
 static void
-add_vary_header_for_compressible_content(TSHttpTxn txnp, bool server, HostConfiguration * /* hc ATS_UNUSED */)
+add_vary_header_for_compressible_content(TSHttpTxn txnp)
 {
   TSMBuffer resp_buf;
   TSMLoc    resp_loc;
 
-  // Get the response headers
-  if (server) {
-    if (TS_SUCCESS != TSHttpTxnServerRespGet(txnp, &resp_buf, &resp_loc)) {
-      return;
-    }
-  } else {
-    if (TS_SUCCESS != TSHttpTxnCachedRespGet(txnp, &resp_buf, &resp_loc)) {
-      return;
-    }
+  if (TS_SUCCESS != TSHttpTxnServerRespGet(txnp, &resp_buf, &resp_loc)) {
+    return;
   }
 
-  // Add Vary: Accept-Encoding header
+  // Add Vary: Accept-Encoding header to the origin response before caching.
   if (vary_header(resp_buf, resp_loc) != TS_SUCCESS) {
     error("failed to add Vary header for compressible content");
     TSHandleMLocRelease(resp_buf, TS_NULL_MLOC, resp_loc);
@@ -798,6 +791,25 @@ add_vary_header_for_compressible_content(TSHttpTxn txnp, bool server, HostConfig
   TSHandleMLocRelease(resp_buf, TS_NULL_MLOC, resp_loc);
 }
 
+static void
+add_vary_header_to_client_response(TSHttpTxn txnp)
+{
+  TSMBuffer resp_buf;
+  TSMLoc    resp_loc;
+
+  if (TS_SUCCESS != TSHttpTxnClientRespGet(txnp, &resp_buf, &resp_loc)) {
+    return;
+  }
+
+  if (vary_header(resp_buf, resp_loc) != TS_SUCCESS) {
+    error("failed to add Vary header to client response");
+    TSHandleMLocRelease(resp_buf, TS_NULL_MLOC, resp_loc);
+    return;
+  }
+
+  TSHandleMLocRelease(resp_buf, TS_NULL_MLOC, resp_loc);
+}
+
 static void
 compress_transform_add(TSHttpTxn txnp, HostConfiguration *hc, int compress_type, int algorithms)
 {
@@ -824,7 +836,7 @@ compress_transform_add(TSHttpTxn txnp, HostConfiguration *hc, int compress_type,
 }
 
 static void
-handle_compression_and_vary(TSHttpTxn txnp, bool server, HostConfiguration *hc, int *compress_type, int *algorithms)
+handle_compression_and_vary(TSCont contp, TSHttpTxn txnp, bool server, HostConfiguration *hc, int *compress_type, int *algorithms)
 {
   // Check if content is compressible and add compression if client accepts it
   bool content_is_compressible;
@@ -834,7 +846,11 @@ handle_compression_and_vary(TSHttpTxn txnp, bool server, HostConfiguration *hc,
 
   // Add Vary: Accept-Encoding for all compressible content to ensure proper HTTP caching
   if (content_is_compressible) {
-    add_vary_header_for_compressible_content(txnp, server, hc);
+    if (server) {
+      add_vary_header_for_compressible_content(txnp);
+    } else {
+      TSHttpTxnHookAdd(txnp, TS_HTTP_SEND_RESPONSE_HDR_HOOK, contp);
+    }
   }
 }
 
@@ -882,7 +898,7 @@ transform_plugin(TSCont contp, TSEvent event, void *edata)
         }
       }
 
-      handle_compression_and_vary(txnp, true, hc, &compress_type, &algorithms);
+      handle_compression_and_vary(contp, txnp, true, hc, &compress_type, &algorithms);
     }
     break;
 
@@ -908,7 +924,7 @@ transform_plugin(TSCont contp, TSEvent event, void *edata)
     if (TS_ERROR != TSHttpTxnCacheLookupStatusGet(txnp, &obj_status) && (TS_CACHE_LOOKUP_HIT_FRESH == obj_status)) {
       if (hc != nullptr) {
         info("handling compression of cached object");
-        handle_compression_and_vary(txnp, false, hc, &compress_type, &algorithms);
+        handle_compression_and_vary(contp, txnp, false, hc, &compress_type, &algorithms);
       }
     } else {
       // Prepare for going to origin
@@ -917,6 +933,10 @@ transform_plugin(TSCont contp, TSEvent event, void *edata)
     }
   } break;
 
+  case TS_EVENT_HTTP_SEND_RESPONSE_HDR:
+    add_vary_header_to_client_response(txnp);
+    break;
+
   case TS_EVENT_HTTP_TXN_CLOSE:
     // Release the ocnif lease, and destroy this continuation
     TSContDestroy(contp);
diff --git a/tests/gold_tests/pluginTest/compress/compress-vary-cached-response.test.py b/tests/gold_tests/pluginTest/compress/compress-vary-cached-response.test.py
new file mode 100644
index 00000000000..033555fc16a
--- /dev/null
+++ b/tests/gold_tests/pluginTest/compress/compress-vary-cached-response.test.py
@@ -0,0 +1,101 @@
+'''
+Regression test for cached Vary header updates when the client omits
+Accept-Encoding.
+'''
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you under the Apache License, Version 2.0 (the
+#  "License"); you may not use this file except in compliance
+#  with the License.  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+Test.Summary = '''
+Regression test for compress Vary header updates on cached responses when the
+client does not send Accept-Encoding.
+'''
+
+Test.SkipUnless(Condition.PluginExists('compress.so'))
+
+server = Test.MakeOriginServer("server")
+request_header = {"headers": "GET /object HTTP/1.1\r\nHost: seed.example\r\n\r\n", "timestamp": "1469733493.993", "body": ""}
+response_header = {
+    "headers":
+        "HTTP/1.1 200 OK\r\n"
+        "Connection: close\r\n"
+        "Cache-Control: public, max-age=3600\r\n"
+        "Content-Type: text/javascript\r\n"
+        "Content-Length: 22\r\n\r\n",
+    "timestamp": "1469733493.993",
+    "body": "var cached_value = 1;\n"
+}
+server.addResponse("sessionlog.json", request_header, response_header)
+
+ts = Test.MakeATSProcess("ts", enable_cache=True)
+ts.Disk.records_config.update(
+    {
+        'proxy.config.diags.debug.enabled': 1,
+        'proxy.config.diags.debug.tags': 'compress',
+        'proxy.config.diags.output.diag': 'L',
+    })
+ts.Setup.Copy("etc/compress-cache-false.config")
+ts.Disk.remap_config.AddLine(f'map http://seed.example/ http://127.0.0.1:{server.Variables.Port}/')
+ts.Disk.remap_config.AddLine(
+    f'map http://compress.example/ http://127.0.0.1:{server.Variables.Port}/'
+    f' @plugin=compress.so @pparam={Test.RunDirectory}/compress-cache-false.config')
+
+ts.Disk.diags_log.Content += Testers.ContainsExpression(
+    'handling compression of cached object', 'compress plugin must process a cached response')
+ts.Disk.diags_log.Content += Testers.ExcludesExpression('cannot add/update the Vary header', 'cached response must not be mutated')
+ts.Disk.diags_log.Content += Testers.ExcludesExpression(
+    'failed to add Vary header for compressible content', 'origin Vary update must succeed')
+ts.Disk.diags_log.Content += Testers.ExcludesExpression(
+    'failed to add Vary header to client response', 'client Vary update must succeed')
+
+seed = Test.AddTestRun('seed cache without compress plugin')
+seed.Processes.Default.StartBefore(server, ready=When.PortOpen(server.Variables.Port))
+seed.Processes.Default.StartBefore(ts)
+seed.Processes.Default.Command = (
+    f'curl --http1.1 -sS -o /dev/null --proxy http://127.0.0.1:{ts.Variables.port}'
+    ' http://seed.example/object')
+seed.Processes.Default.ReturnCode = 0
+seed.StillRunningAfter = server
+seed.StillRunningAfter = ts
+
+first_cached = Test.AddTestRun('add Vary to cached client response')
+first_cached.Processes.Default.Command = (
+    f'curl --http1.1 -sS -o /dev/null --proxy http://127.0.0.1:{ts.Variables.port}'
+    ' http://compress.example/object')
+first_cached.Processes.Default.ReturnCode = 0
+first_cached.StillRunningAfter = server
+first_cached.StillRunningAfter = ts
+
+headers_path = f'{Test.RunDirectory}/cached_headers.txt'
+body_path = f'{Test.RunDirectory}/cached_body.txt'
+second_cached = Test.AddTestRun('verify cached client response Vary header')
+second_cached.Processes.Default.Command = (
+    f'curl --http1.1 -sS -D {headers_path} -o {body_path}'
+    f' --proxy http://127.0.0.1:{ts.Variables.port}'
+    ' http://compress.example/object')
+second_cached.Processes.Default.ReturnCode = 0
+second_cached.StillRunningAfter = server
+second_cached.StillRunningAfter = ts
+
+verify = Test.AddTestRun('verify cached response headers and body')
+verify.Processes.Default.Command = (
+    f"grep -i '^Vary:.*Accept-Encoding' {headers_path}"
+    f" && ! grep -i '^Content-Encoding:' {headers_path}"
+    f" && diff {body_path} - <<'EOF'\n"
+    'var cached_value = 1;\n'
+    'EOF')
+verify.Processes.Default.ReturnCode = 0
+verify.StillRunningAfter = server
+verify.StillRunningAfter = ts