diff --git a/http2/clientconn_test.go b/http2/clientconn_test.go
index 0f57d3e3..e587421e 100644
--- a/http2/clientconn_test.go
+++ b/http2/clientconn_test.go
@@ -437,7 +437,7 @@ func (rt *testRoundTrip) wantStatus(want int) {
 	}
 }
 
-// body reads the contents of the response body.
+// readBody reads the contents of the response body.
 func (rt *testRoundTrip) readBody() ([]byte, error) {
 	t := rt.t
 	t.Helper()
diff --git a/internal/http3/roundtrip.go b/internal/http3/roundtrip.go
index d52c8455..58253d0b 100644
--- a/internal/http3/roundtrip.go
+++ b/internal/http3/roundtrip.go
@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"sync"
 
+	"golang.org/x/net/http/httpguts"
 	"golang.org/x/net/internal/httpcommon"
 )
 
@@ -113,14 +114,18 @@ func (cc *ClientConn) RoundTrip(req *http.Request) (_ *http.Response, err error)
 		return nil, err
 	}
 
+	is100ContinueReq := httpguts.HeaderValuesContainsToken(req.Header["Expect"], "100-continue")
 	if encr.HasBody {
-		// TODO: Defer sending the request body when "Expect: 100-continue" is set.
 		rt.reqBody = req.Body
 		rt.reqBodyWriter.st = st
 		rt.reqBodyWriter.remain = contentLength
 		rt.reqBodyWriter.flush = true
 		rt.reqBodyWriter.name = "request"
-		go copyRequestBody(rt)
+
+		if !is100ContinueReq {
+			encr.HasBody = false
+			go copyRequestBody(rt)
+		}
 	}
 
 	// Read the response headers.
@@ -138,7 +143,19 @@ func (cc *ClientConn) RoundTrip(req *http.Request) (_ *http.Response, err error)
 
 			if statusCode >= 100 && statusCode < 199 {
 				// TODO: Handle 1xx responses.
-				continue
+				switch statusCode {
+				case 100:
+					if encr.HasBody && is100ContinueReq {
+						encr.HasBody = false
+						go copyRequestBody(rt)
+						continue
+					}
+					// If we did not send "Expect: 100-continue" request but
+					// received status 100 anyways, just continue per usual and
+					// let the caller decide what to do with the response.
+				default:
+					continue
+				}
 			}
 
 			// We have the response headers.
diff --git a/internal/http3/roundtrip_test.go b/internal/http3/roundtrip_test.go
index 230ff82c..efbe1052 100644
--- a/internal/http3/roundtrip_test.go
+++ b/internal/http3/roundtrip_test.go
@@ -352,3 +352,70 @@ func TestRoundTripRequestBodyErrorAfterHeaders(t *testing.T) {
 		}
 	})
 }
+
+func TestRoundTripExpect100Continue(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		tc := newTestClientConn(t)
+		tc.greet()
+		clientBody := []byte("client's body that will be sent later")
+		serverBody := []byte("server's body")
+
+		// Client sends an Expect: 100-continue request.
+		req, _ := http.NewRequest("PUT", "https://example.tld/", bytes.NewBuffer(clientBody))
+		req.Header = http.Header{"Expect": {"100-continue"}}
+		rt := tc.roundTrip(req)
+		st := tc.wantStream(streamTypeRequest)
+
+		// Server reads the header.
+		st.wantHeaders(nil)
+		st.wantIdle("client has yet to send its body")
+
+		// Server responds with HTTP status 100.
+		st.writeHeaders(http.Header{
+			":status": []string{"100"},
+		})
+
+		// Client sends its body after receiving HTTP status 100 response.
+		st.wantData(clientBody)
+
+		// The server sends its response after getting the client's body.
+		st.writeHeaders(http.Header{
+			":status": []string{"200"},
+		})
+		st.writeData(serverBody)
+		st.stream.stream.CloseWrite()
+
+		// Client receives the response from server.
+		rt.wantStatus(200)
+		rt.wantBody(serverBody)
+	})
+}
+
+func TestRoundTripExpect100ContinueRejected(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		tc := newTestClientConn(t)
+		tc.greet()
+
+		// Client sends an Expect: 100-continue request.
+		req, _ := http.NewRequest("PUT", "https://example.tld/", bytes.NewBufferString("client's body"))
+		req.Header = http.Header{"Expect": {"100-continue"}}
+		rt := tc.roundTrip(req)
+		st := tc.wantStream(streamTypeRequest)
+
+		// Server reads the header.
+		st.wantHeaders(nil)
+		st.wantIdle("client has yet to send its body")
+
+		// Server rejects it.
+		st.writeHeaders(http.Header{
+			":status": []string{"200"},
+		})
+		st.wantIdle("client does not send its body without getting status 100")
+		serverBody := []byte("server's body")
+		st.writeData(serverBody)
+		st.stream.stream.CloseWrite()
+
+		rt.wantStatus(200)
+		rt.wantBody(serverBody)
+	})
+}
diff --git a/internal/http3/transport_test.go b/internal/http3/transport_test.go
index 271cf262..c9858e4a 100644
--- a/internal/http3/transport_test.go
+++ b/internal/http3/transport_test.go
@@ -454,6 +454,26 @@ func (rt *testRoundTrip) wantHeaders(want http.Header) {
 	}
 }
 
+// readBody reads the contents of the response body.
+func (rt *testRoundTrip) readBody() ([]byte, error) {
+	t := rt.t
+	t.Helper()
+	return io.ReadAll(rt.response().Body)
+}
+
+// wantBody consumes the a body and asserts that it is as expected.
+func (rt *testRoundTrip) wantBody(want []byte) {
+	t := rt.t
+	t.Helper()
+	got, err := rt.readBody()
+	if err != nil {
+		t.Fatalf("unexpected error reading response body: %v", err)
+	}
+	if !bytes.Equal(got, want) {
+		t.Fatalf("unexpected response body:\ngot:  %q\nwant: %q", got, want)
+	}
+}
+
 func (tc *testClientConn) roundTrip(req *http.Request) *testRoundTrip {
 	rt := &testRoundTrip{t: tc.t}
 	go func() {