diff --git a/http3/client.go b/http3/client.go
index b0e6546c..b2b7bbdc 100644
--- a/http3/client.go
+++ b/http3/client.go
@@ -327,31 +327,43 @@ func (c *client) RoundTripOpt(req *http.Request, opt RoundTripOpt) (*http.Respon
 	return rsp, rerr.err
 }
 
-func (c *client) sendRequestBody(str Stream, body io.ReadCloser) error {
-	defer body.Close()
-	b := make([]byte, bodyCopyBufferSize)
-	for {
-		n, rerr := body.Read(b)
-		if n == 0 {
-			if rerr == nil {
-				continue
-			}
-			if rerr == io.EOF {
-				break
-			}
-		}
-		if _, err := str.Write(b[:n]); err != nil {
-			return err
-		}
-		if rerr != nil {
-			if rerr == io.EOF {
-				break
-			}
-			str.CancelWrite(quic.StreamErrorCode(ErrCodeRequestCanceled))
-			return rerr
-		}
+// cancelingReader reads from the io.Reader.
+// It cancels writing on the stream if any error other than io.EOF occurs.
+type cancelingReader struct {
+	r   io.Reader
+	str Stream
+}
+
+func (r *cancelingReader) Read(b []byte) (int, error) {
+	n, err := r.r.Read(b)
+	if err != nil && err != io.EOF {
+		r.str.CancelWrite(quic.StreamErrorCode(ErrCodeRequestCanceled))
 	}
-	return nil
+	return n, err
+}
+
+func (c *client) sendRequestBody(str Stream, body io.ReadCloser, contentLength int64) error {
+	defer body.Close()
+	buf := make([]byte, bodyCopyBufferSize)
+	sr := &cancelingReader{str: str, r: body}
+	if contentLength == -1 {
+		_, err := io.CopyBuffer(str, sr, buf)
+		return err
+	}
+
+	// make sure we don't send more bytes than the content length
+	n, err := io.CopyBuffer(str, io.LimitReader(sr, contentLength), buf)
+	if err != nil {
+		return err
+	}
+	var extra int64
+	extra, err = io.CopyBuffer(io.Discard, sr, buf)
+	n += extra
+	if n > contentLength {
+		str.CancelWrite(quic.StreamErrorCode(ErrCodeRequestCanceled))
+		return fmt.Errorf("http: ContentLength=%d with Body length %d", contentLength, n)
+	}
+	return err
 }
 
 func (c *client) doRequest(req *http.Request, conn quic.EarlyConnection, str quic.Stream, opt RoundTripOpt, reqDone chan<- struct{}) (*http.Response, requestError) {
@@ -371,7 +383,13 @@ func (c *client) doRequest(req *http.Request, conn quic.EarlyConnection, str qui
 	if req.Body != nil {
 		// send the request body asynchronously
 		go func() {
-			if err := c.sendRequestBody(hstr, req.Body); err != nil {
+			contentLength := int64(-1)
+			// According to the documentation for http.Request.ContentLength,
+			// a value of 0 with a non-nil Body is also treated as unknown content length.
+			if req.ContentLength > 0 {
+				contentLength = req.ContentLength
+			}
+			if err := c.sendRequestBody(hstr, req.Body, contentLength); err != nil {
 				c.logger.Errorf("Error writing request: %s", err)
 			}
 			if !opt.DontCloseRequestStream {
diff --git a/http3/client_test.go b/http3/client_test.go
index ebcfa3bc..1c5ac6f8 100644
--- a/http3/client_test.go
+++ b/http3/client_test.go
@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"sync"
 	"time"
 
 	"github.com/quic-go/quic-go"
@@ -803,6 +804,29 @@ var _ = Describe("Client", func() {
 				Expect(hfs).To(HaveKeyWithValue(":path", "/upload"))
 			})
 
+			It("doesn't send more bytes than allowed by http.Request.ContentLength", func() {
+				req.ContentLength = 7
+				var once sync.Once
+				done := make(chan struct{})
+				gomock.InOrder(
+					str.EXPECT().CancelWrite(gomock.Any()).Do(func(c quic.StreamErrorCode) {
+						once.Do(func() {
+							Expect(c).To(Equal(quic.StreamErrorCode(ErrCodeRequestCanceled)))
+							close(done)
+						})
+					}).AnyTimes(),
+					str.EXPECT().Close().MaxTimes(1),
+					str.EXPECT().CancelWrite(gomock.Any()).AnyTimes(),
+				)
+				str.EXPECT().Read(gomock.Any()).DoAndReturn(func([]byte) (int, error) {
+					<-done
+					return 0, errors.New("done")
+				})
+				cl.RoundTripOpt(req, RoundTripOpt{})
+				Expect(strBuf.String()).To(ContainSubstring("request"))
+				Expect(strBuf.String()).ToNot(ContainSubstring("request body"))
+			})
+
 			It("returns the error that occurred when reading the body", func() {
 				req.Body.(*mockBody).readErr = errors.New("testErr")
 				done := make(chan struct{})