diff --git a/src/h2load.cc b/src/h2load.cc
index 0cbcaebb..a51b2372 100644
--- a/src/h2load.cc
+++ b/src/h2load.cc
@@ -34,6 +34,7 @@
 #ifdef HAVE_FCNTL_H
 #  include <fcntl.h>
 #endif // HAVE_FCNTL_H
+#include <sys/mman.h>
 
 #include <cstdio>
 #include <cassert>
@@ -90,6 +91,7 @@ Config::Config()
                     "CHACHA20_POLY1305_SHA256:TLS_AES_128_CCM_SHA256"),
       groups("X25519:P-256:P-384:P-521"),
       data_length(-1),
+      data(nullptr),
       addrs(nullptr),
       nreqs(1),
       nclients(1),
@@ -2661,6 +2663,13 @@ int main(int argc, char **argv) {
       exit(EXIT_FAILURE);
     }
     config.data_length = data_stat.st_size;
+    auto addr = mmap(nullptr, config.data_length, PROT_READ, MAP_SHARED,
+                     config.data_fd, 0);
+    if (addr == MAP_FAILED) {
+      std::cerr << "-d: Could not mmap file " << datafile << std::endl;
+      exit(EXIT_FAILURE);
+    }
+    config.data = static_cast<uint8_t *>(addr);
   }
 
   if (!logfile.empty()) {
diff --git a/src/h2load.h b/src/h2load.h
index fdacf727..3f51c2c8 100644
--- a/src/h2load.h
+++ b/src/h2load.h
@@ -81,6 +81,8 @@ struct Config {
   std::string groups;
   // length of upload data
   int64_t data_length;
+  // memory mapped upload data
+  uint8_t *data;
   addrinfo *addrs;
   size_t nreqs;
   size_t nclients;
diff --git a/src/h2load_http3_session.cc b/src/h2load_http3_session.cc
index 3fad5dc7..24e99b9d 100644
--- a/src/h2load_http3_session.cc
+++ b/src/h2load_http3_session.cc
@@ -64,6 +64,29 @@ int Http3Session::submit_request() {
   return 0;
 }
 
+namespace {
+nghttp3_ssize read_data(nghttp3_conn *conn, int64_t stream_id, nghttp3_vec *vec,
+                        size_t veccnt, uint32_t *pflags, void *user_data,
+                        void *stream_user_data) {
+  auto s = static_cast<Http3Session *>(user_data);
+
+  s->read_data(vec, veccnt, pflags);
+
+  return 1;
+}
+} // namespace
+
+void Http3Session::read_data(nghttp3_vec *vec, size_t veccnt,
+                             uint32_t *pflags) {
+  assert(veccnt > 0);
+
+  auto config = client_->worker->config;
+
+  vec[0].base = config->data;
+  vec[0].len = config->data_length;
+  *pflags |= NGHTTP3_DATA_FLAG_EOF;
+}
+
 int64_t Http3Session::submit_request_internal() {
   int rv;
   int64_t stream_id;
@@ -76,9 +99,12 @@ int64_t Http3Session::submit_request_internal() {
     return rv;
   }
 
-  rv = nghttp3_conn_submit_request(conn_, stream_id,
-                                   reinterpret_cast<nghttp3_nv *>(nva.data()),
-                                   nva.size(), nullptr, nullptr);
+  nghttp3_data_reader dr{};
+  dr.read_data = h2load::read_data;
+
+  rv = nghttp3_conn_submit_request(
+      conn_, stream_id, reinterpret_cast<nghttp3_nv *>(nva.data()), nva.size(),
+      config->data_fd == -1 ? nullptr : &dr, nullptr);
   if (rv != 0) {
     return rv;
   }
diff --git a/src/h2load_http3_session.h b/src/h2load_http3_session.h
index cd3d96ba..aaca13dc 100644
--- a/src/h2load_http3_session.h
+++ b/src/h2load_http3_session.h
@@ -66,6 +66,8 @@ public:
   int add_write_offset(int64_t stream_id, size_t ndatalen);
   int add_ack_offset(int64_t stream_id, size_t datalen);
 
+  void read_data(nghttp3_vec *vec, size_t veccnt, uint32_t *pflags);
+
 private:
   Client *client_;
   nghttp3_conn *conn_;