diff --git a/auth/client.go b/auth/client.go
index 7c65e9de..3ed6b695 100644
--- a/auth/client.go
+++ b/auth/client.go
@@ -7,8 +7,10 @@
 package auth
 
 import (
+	"bytes"
 	"context"
 	"errors"
+	"io"
 	"net/http"
 	"sync"
 
@@ -67,6 +69,28 @@ func (t *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	base := t.opts.Base
 	t.mu.Unlock()
 
+	var (
+		// If haveBody is set, the request has a nontrivial body, and we need avoid
+		// reading (or closing) it multiple times. In that case, bodyBytes is its
+		// content.
+		haveBody  bool
+		bodyBytes []byte
+	)
+	if req.Body != nil && req.Body != http.NoBody {
+		// if we're setting Body, we must mutate first.
+		req = req.Clone(req.Context())
+		haveBody = true
+		var err error
+		bodyBytes, err = io.ReadAll(req.Body)
+		if err != nil {
+			return nil, err
+		}
+		// Now that we've read the request body, http.RoundTripper requires that we
+		// close it.
+		req.Body.Close() // ignore error
+		req.Body = io.NopCloser(bytes.NewReader(bodyBytes))
+	}
+
 	resp, err := base.RoundTrip(req)
 	if err != nil {
 		return nil, err
@@ -97,7 +121,15 @@ func (t *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 		}
 		t.opts.Base = &oauth2.Transport{Base: t.opts.Base, Source: ts}
 	}
-	return t.opts.Base.RoundTrip(req.Clone(req.Context()))
+
+	// If we don't have a body, the request is reusable, though it will be cloned
+	// by the base. However, if we've had to read the body, we must clone.
+	if haveBody {
+		req = req.Clone(req.Context())
+		req.Body = io.NopCloser(bytes.NewReader(bodyBytes))
+	}
+
+	return t.opts.Base.RoundTrip(req)
 }
 
 func extractResourceMetadataURL(authHeaders []string) string {
diff --git a/auth/client_test.go b/auth/client_test.go
index 310fc56e..e1b7dc70 100644
--- a/auth/client_test.go
+++ b/auth/client_test.go
@@ -10,13 +10,24 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"golang.org/x/oauth2"
 )
 
+// A basicReader is an io.Reader to be used as a non-rereadable request body.
+//
+// net/http has special handling for strings.Reader that we want to avoid.
+type basicReader struct {
+	r *strings.Reader
+}
+
+func (r *basicReader) Read(p []byte) (n int, err error) { return r.r.Read(p) }
+
 // TestHTTPTransport validates the OAuth HTTPTransport.
 func TestHTTPTransport(t *testing.T) {
 	const testToken = "test-token-123"
@@ -27,6 +38,20 @@ func TestHTTPTransport(t *testing.T) {
 
 	// authServer simulates a resource that requires OAuth.
 	authServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == http.MethodPost {
+			// Ensure that the body was properly cloned, by reading it completely.
+			// If the body is not cloned, reading it the second time may yield no
+			// bytes.
+			body, err := io.ReadAll(r.Body)
+			if err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+			if len(body) == 0 {
+				http.Error(w, "empty body", http.StatusBadRequest)
+				return
+			}
+		}
 		authHeader := r.Header.Get("Authorization")
 		if authHeader == fmt.Sprintf("Bearer %s", testToken) {
 			w.WriteHeader(http.StatusOK)
@@ -82,6 +107,31 @@ func TestHTTPTransport(t *testing.T) {
 		}
 	})
 
+	t.Run("request body is cloned", func(t *testing.T) {
+		handler := func(ctx context.Context, args OAuthHandlerArgs) (oauth2.TokenSource, error) {
+			if args.ResourceMetadataURL != "http://metadata.example.com" {
+				t.Errorf("handler got metadata URL %q, want %q", args.ResourceMetadataURL, "http://metadata.example.com")
+			}
+			return fakeTokenSource, nil
+		}
+
+		transport, err := NewHTTPTransport(handler, nil)
+		if err != nil {
+			t.Fatalf("NewHTTPTransport() failed: %v", err)
+		}
+		client := &http.Client{Transport: transport}
+
+		resp, err := client.Post(authServer.URL, "application/json", &basicReader{strings.NewReader("{}")})
+		if err != nil {
+			t.Fatalf("client.Post() failed: %v", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("got status %d, want %d", resp.StatusCode, http.StatusOK)
+		}
+	})
+
 	t.Run("handler returns error", func(t *testing.T) {
 		handlerErr := errors.New("user rejected auth")
 		handler := func(ctx context.Context, args OAuthHandlerArgs) (oauth2.TokenSource, error) {