diff --git a/src/io/multi.go b/src/io/multi.go
index 24ee71e4ca..909b7e4523 100644
--- a/src/io/multi.go
+++ b/src/io/multi.go
@@ -41,6 +41,31 @@ func (mr *multiReader) Read(p []byte) (n int, err error) {
 	return 0, EOF
 }
 
+func (mr *multiReader) WriteTo(w Writer) (sum int64, err error) {
+	return mr.writeToWithBuffer(w, make([]byte, 1024 * 32))
+}
+
+func (mr *multiReader) writeToWithBuffer(w Writer, buf []byte) (sum int64, err error) {
+	for i, r := range mr.readers {
+		var n int64
+		if subMr, ok := r.(*multiReader); ok { // reuse buffer with nested multiReaders
+			n, err = subMr.writeToWithBuffer(w, buf)
+		} else {
+			n, err = copyBuffer(w, r, buf)
+		}
+		sum += n
+		if err != nil {
+			mr.readers = mr.readers[i:] // permit resume / retry after error
+			return sum, err
+		}
+		mr.readers[i] = nil // permit early GC
+	}
+	mr.readers = nil
+	return sum, nil
+}
+
+var _ WriterTo = (*multiReader)(nil)
+
 // MultiReader returns a Reader that's the logical concatenation of
 // the provided input readers. They're read sequentially. Once all
 // inputs have returned EOF, Read will return EOF.  If any of the readers
diff --git a/src/io/multi_test.go b/src/io/multi_test.go
index e877e54571..679312c23b 100644
--- a/src/io/multi_test.go
+++ b/src/io/multi_test.go
@@ -63,6 +63,31 @@ func TestMultiReader(t *testing.T) {
 	})
 }
 
+func TestMultiReaderAsWriterTo(t *testing.T) {
+	mr := MultiReader(
+		strings.NewReader("foo "),
+		MultiReader( // Tickle the buffer reusing codepath
+			strings.NewReader(""),
+			strings.NewReader("bar"),
+		),
+	)
+	mrAsWriterTo, ok := mr.(WriterTo)
+	if !ok {
+		t.Fatalf("expected cast to WriterTo to succeed")
+	}
+	sink := &strings.Builder{}
+	n, err := mrAsWriterTo.WriteTo(sink)
+	if err != nil {
+		t.Fatalf("expected no error; got %v", err)
+	}
+	if n != 7 {
+		t.Errorf("expected read 7 bytes; got %d", n)
+	}
+	if result := sink.String(); result != "foo bar" {
+		t.Errorf(`expected "foo bar"; got %q`, result)
+	}
+}
+
 func TestMultiWriter(t *testing.T) {
 	sink := new(bytes.Buffer)
 	// Hide bytes.Buffer's WriteString method: