diff --git a/gennghttpxfun.py b/gennghttpxfun.py
index af8d60a5..6f88456e 100755
--- a/gennghttpxfun.py
+++ b/gennghttpxfun.py
@@ -197,6 +197,7 @@ OPTIONS = [
     "rlimit-memlock",
     "max-worker-processes",
     "worker-process-grace-shutdown-period",
+    "frontend-quic-initial-rtt",
 ]
 
 LOGVARS = [
diff --git a/src/shrpx.cc b/src/shrpx.cc
index 1cdc2c59..a36c168b 100644
--- a/src/shrpx.cc
+++ b/src/shrpx.cc
@@ -1958,6 +1958,8 @@ void fill_default_config(Config *config) {
       assert(0);
       abort();
     }
+
+    upstreamconf.initial_rtt = NGTCP2_DEFAULT_INITIAL_RTT;
   }
 
   auto &http3conf = config->http3;
@@ -3399,6 +3401,13 @@ HTTP/3 and QUIC:
               (which is 8  bytes long).  If this option  is omitted, a
               random   server  ID   is   generated   on  startup   and
               configuration reload.
+  --frontend-quic-initial-rtt=<DURATION>
+              Specify the initial RTT of the frontend QUIC connection.
+              Default: )"
+      << util::duration_str(
+             static_cast<double>(config->quic.upstream.initial_rtt) /
+             NGTCP2_SECONDS)
+      << R"(
   --no-quic-bpf
               Disable eBPF.
   --frontend-http3-window-size=<SIZE>
@@ -4218,6 +4227,8 @@ int main(int argc, char **argv) {
         {SHRPX_OPT_MAX_WORKER_PROCESSES.c_str(), required_argument, &flag, 188},
         {SHRPX_OPT_WORKER_PROCESS_GRACE_SHUTDOWN_PERIOD.c_str(),
          required_argument, &flag, 189},
+        {SHRPX_OPT_FRONTEND_QUIC_INITIAL_RTT.c_str(), required_argument, &flag,
+         190},
         {nullptr, 0, nullptr, 0}};
 
     int option_index = 0;
@@ -5118,6 +5129,11 @@ int main(int argc, char **argv) {
         cmdcfgs.emplace_back(SHRPX_OPT_WORKER_PROCESS_GRACE_SHUTDOWN_PERIOD,
                              StringRef{optarg});
         break;
+      case 190:
+        // --frontend-quic-initial-rtt
+        cmdcfgs.emplace_back(SHRPX_OPT_FRONTEND_QUIC_INITIAL_RTT,
+                             StringRef{optarg});
+        break;
       default:
         break;
       }
diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index ac260b55..25576c70 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -2440,6 +2440,11 @@ int option_lookup_token(const char *name, size_t namelen) {
         return SHRPX_OPTID_MAX_REQUEST_HEADER_FIELDS;
       }
       break;
+    case 't':
+      if (util::strieq_l("frontend-quic-initial-rt", name, 24)) {
+        return SHRPX_OPTID_FRONTEND_QUIC_INITIAL_RTT;
+      }
+      break;
     }
     break;
   case 26:
@@ -4152,6 +4157,19 @@ int parse_config(Config *config, int optid, const StringRef &opt,
   case SHRPX_OPTID_WORKER_PROCESS_GRACE_SHUTDOWN_PERIOD:
     return parse_duration(&config->worker_process_grace_shutdown_period, opt,
                           optarg);
+  case SHRPX_OPTID_FRONTEND_QUIC_INITIAL_RTT: {
+#ifdef ENABLE_HTTP3
+    ev_tstamp d;
+    if (parse_duration(&d, opt, optarg) != 0) {
+      return -1;
+    }
+
+    config->quic.upstream.initial_rtt =
+        static_cast<ngtcp2_duration>(d * NGTCP2_SECONDS);
+#endif // ENABLE_HTTP3
+
+    return 0;
+  }
   case SHRPX_OPTID_CONF:
     LOG(WARN) << "conf: ignored";
 
diff --git a/src/shrpx_config.h b/src/shrpx_config.h
index 8ae8c26c..1b19d2f5 100644
--- a/src/shrpx_config.h
+++ b/src/shrpx_config.h
@@ -400,6 +400,8 @@ constexpr auto SHRPX_OPT_MAX_WORKER_PROCESSES =
     StringRef::from_lit("max-worker-processes");
 constexpr auto SHRPX_OPT_WORKER_PROCESS_GRACE_SHUTDOWN_PERIOD =
     StringRef::from_lit("worker-process-grace-shutdown-period");
+constexpr auto SHRPX_OPT_FRONTEND_QUIC_INITIAL_RTT =
+    StringRef::from_lit("frontend-quic-initial-rtt");
 
 constexpr size_t SHRPX_OBFUSCATED_NODE_LENGTH = 8;
 
@@ -780,6 +782,7 @@ struct QUICConfig {
     bool require_token;
     std::array<uint8_t, SHRPX_QUIC_SERVER_IDLEN> server_id;
     StringRef secret_file;
+    ngtcp2_duration initial_rtt;
   } upstream;
   struct {
     StringRef prog_file;
@@ -1242,6 +1245,7 @@ enum {
   SHRPX_OPTID_FRONTEND_QUIC_DEBUG_LOG,
   SHRPX_OPTID_FRONTEND_QUIC_EARLY_DATA,
   SHRPX_OPTID_FRONTEND_QUIC_IDLE_TIMEOUT,
+  SHRPX_OPTID_FRONTEND_QUIC_INITIAL_RTT,
   SHRPX_OPTID_FRONTEND_QUIC_QLOG_DIR,
   SHRPX_OPTID_FRONTEND_QUIC_REQUIRE_TOKEN,
   SHRPX_OPTID_FRONTEND_QUIC_SECRET_FILE,
diff --git a/src/shrpx_http3_upstream.cc b/src/shrpx_http3_upstream.cc
index aa0d1f8c..f59d0fff 100644
--- a/src/shrpx_http3_upstream.cc
+++ b/src/shrpx_http3_upstream.cc
@@ -592,6 +592,7 @@ int Http3Upstream::init(const UpstreamAddr *faddr, const Address &remote_addr,
   }
 
   settings.initial_ts = quic_timestamp();
+  settings.initial_rtt = quicconf.upstream.initial_rtt;
   settings.cc_algo = quicconf.upstream.congestion_controller;
   settings.max_window = http3conf.upstream.max_connection_window_size;
   settings.max_stream_window = http3conf.upstream.max_window_size;