diff --git a/src/node_http2.cc b/src/node_http2.cc
index 7170907ced1b3e..0c0417baf17130 100644
--- a/src/node_http2.cc
+++ b/src/node_http2.cc
@@ -1872,7 +1872,10 @@ void Http2Session::OnStreamRead(ssize_t nread, const uv_buf_t& buf_) {
     nread = buf.size();
     stream_buf_offset_ = 0;
     stream_buf_ab_.Reset();
-    DecrementCurrentSessionMemory(stream_buf_offset_);
+
+    // We have now fully processed the stream_buf_ input chunk (by moving the
+    // remaining part into buf, which will be accounted for below).
+    DecrementCurrentSessionMemory(stream_buf_.len);
   }
 
   // Shrink to the actual amount of used data.
diff --git a/test/parallel/test-http2-large-writes-session-memory-leak.js b/test/parallel/test-http2-large-writes-session-memory-leak.js
new file mode 100644
index 00000000000000..641923c06c9133
--- /dev/null
+++ b/test/parallel/test-http2-large-writes-session-memory-leak.js
@@ -0,0 +1,55 @@
+'use strict';
+const common = require('../common');
+if (!common.hasCrypto)
+  common.skip('missing crypto');
+const fixtures = require('../common/fixtures');
+const http2 = require('http2');
+
+// Regression test for https://github.com/nodejs/node/issues/29223.
+// There was a "leak" in the accounting of session memory leading
+// to streams eventually failing with NGHTTP2_ENHANCE_YOUR_CALM.
+
+const server = http2.createSecureServer({
+  key: fixtures.readKey('agent2-key.pem'),
+  cert: fixtures.readKey('agent2-cert.pem'),
+});
+
+// Simple server that sends 200k and closes the stream.
+const data200k = 'a'.repeat(200 * 1024);
+server.on('stream', (stream) => {
+  stream.write(data200k);
+  stream.end();
+});
+
+server.listen(0, common.mustCall(() => {
+  const client = http2.connect(`https://localhost:${server.address().port}`, {
+    ca: fixtures.readKey('agent2-cert.pem'),
+    servername: 'agent2',
+
+    // Set maxSessionMemory to 1MB so the leak causes errors faster.
+    maxSessionMemory: 1
+  });
+
+  // Repeatedly create a new stream and read the incoming data. Even though we
+  // only have one stream active at a time, prior to the fix for #29223,
+  // session memory would steadily increase and we'd eventually hit the 1MB
+  // maxSessionMemory limit and get NGHTTP2_ENHANCE_YOUR_CALM errors trying to
+  // create new streams.
+  let streamsLeft = 50;
+  function newStream() {
+    const stream = client.request({ ':path': '/' });
+
+    stream.on('data', () => { });
+
+    stream.on('close', () => {
+      if (streamsLeft-- > 0) {
+        newStream();
+      } else {
+        client.destroy();
+        server.close();
+      }
+    });
+  }
+
+  newStream();
+}));