diff --git a/transport/http/middleware_content_length.go b/transport/http/middleware_content_length.go
new file mode 100644
index 000000000..20586aad6
--- /dev/null
+++ b/transport/http/middleware_content_length.go
@@ -0,0 +1,50 @@
+package http
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+
+	"github.com/awslabs/smithy-go/middleware"
+)
+
+// ContentLengthMiddleware provides a middleware to set the content-length
+// header for the length of a serialize request body.
+type ContentLengthMiddleware struct {
+}
+
+// AddContentLengthMiddleware adds ContentLengthMiddleware to the middleware
+// stack's Build step.
+func AddContentLengthMiddleware(stack *middleware.Stack) {
+	stack.Build.Add(&ContentLengthMiddleware{}, middleware.After)
+}
+
+// ID the identifier for the ContentLengthMiddleware
+func (m *ContentLengthMiddleware) ID() string { return "ContentLengthMiddleware" }
+
+// HandleBuild adds the length of the serialized request to the HTTP header
+// if the length can be determined.
+func (m *ContentLengthMiddleware) HandleBuild(
+	ctx context.Context, in middleware.BuildInput, next middleware.BuildHandler,
+) (
+	out middleware.BuildOutput, metadata middleware.Metadata, err error,
+) {
+	req, ok := in.Request.(*Request)
+	if !ok {
+		return out, metadata, fmt.Errorf("unknown request type %T", req)
+	}
+
+	// Don't set content length if header is already set.
+	if vs := req.Header.Values("Content-Length"); len(vs) != 0 {
+		return next.HandleBuild(ctx, in)
+	}
+
+	if n, ok, err := req.StreamLength(); err != nil {
+		return out, metadata, fmt.Errorf(
+			"failed getting length of request stream, %w", err)
+	} else if ok {
+		req.Header.Set("Content-Length", strconv.FormatInt(n, 10))
+	}
+
+	return next.HandleBuild(ctx, in)
+}
diff --git a/transport/http/middleware_content_length_test.go b/transport/http/middleware_content_length_test.go
new file mode 100644
index 000000000..418b6ee22
--- /dev/null
+++ b/transport/http/middleware_content_length_test.go
@@ -0,0 +1,132 @@
+package http
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/awslabs/smithy-go/middleware"
+)
+
+func TestContentLengthMiddleware(t *testing.T) {
+	cases := map[string]struct {
+		Stream    io.Reader
+		ExpectLen string
+		ExpectErr string
+	}{
+		// Cases
+		"bytes.Reader": {
+			Stream:    bytes.NewReader(make([]byte, 10)),
+			ExpectLen: "10",
+		},
+		"bytes.Buffer": {
+			Stream:    bytes.NewBuffer(make([]byte, 10)),
+			ExpectLen: "10",
+		},
+		"strings.Reader": {
+			Stream:    strings.NewReader("hello"),
+			ExpectLen: "5",
+		},
+		"un-seekable and no length": {
+			Stream: &basicReader{buf: make([]byte, 10)},
+		},
+		"with error": {
+			Stream:    &errorSecondSeekableReader{err: fmt.Errorf("seek failed")},
+			ExpectErr: "seek failed",
+		},
+	}
+
+	for name, c := range cases {
+		t.Run(name, func(t *testing.T) {
+			var err error
+			req := NewStackRequest().(*Request)
+			req, err = req.SetStream(c.Stream)
+			if err != nil {
+				t.Fatalf("expect to set stream, %v", err)
+			}
+
+			var m ContentLengthMiddleware
+			_, _, err = m.HandleBuild(context.Background(),
+				middleware.BuildInput{Request: req},
+				nopBuildHandler{},
+			)
+			if len(c.ExpectErr) != 0 {
+				if err == nil {
+					t.Fatalf("expect error, got none")
+				}
+				if e, a := c.ExpectErr, err.Error(); !strings.Contains(a, e) {
+					t.Fatalf("expect error to contain %q, got %v", e, a)
+				}
+			} else if err != nil {
+				t.Fatalf("expect no error, got %v", err)
+			}
+
+			t.Logf("request Content-Length:%v", req.Header.Get("Content-Length"))
+
+			if e, a := c.ExpectLen, req.Header.Get("Content-Length"); e != a {
+				t.Errorf("expect %v content-length, got %v", e, a)
+			}
+		})
+	}
+}
+
+func TestContentLengthMiddleware_HeaderSet(t *testing.T) {
+	req := NewStackRequest().(*Request)
+	req.Header.Set("Content-Length", "1234")
+
+	var err error
+	req, err = req.SetStream(strings.NewReader("hello"))
+	if err != nil {
+		t.Fatalf("expect to set stream, %v", err)
+	}
+
+	var m ContentLengthMiddleware
+	_, _, err = m.HandleBuild(context.Background(),
+		middleware.BuildInput{Request: req},
+		nopBuildHandler{},
+	)
+	if err != nil {
+		t.Fatalf("expect middleware to run, %v", err)
+	}
+
+	if e, a := "1234", req.Header.Get("Content-Length"); e != a {
+		t.Errorf("expect Content-Length not to change, got %v", a)
+	}
+}
+
+type nopBuildHandler struct{}
+
+func (nopBuildHandler) HandleBuild(ctx context.Context, in middleware.BuildInput) (
+	out middleware.BuildOutput, metadata middleware.Metadata, err error,
+) {
+	return out, metadata, nil
+}
+
+type basicReader struct {
+	buf []byte
+}
+
+func (r *basicReader) Read(p []byte) (int, error) {
+	n := copy(p, r.buf)
+	r.buf = r.buf[n:]
+	return n, nil
+}
+
+type errorSecondSeekableReader struct {
+	err   error
+	count int
+}
+
+func (r *errorSecondSeekableReader) Read(p []byte) (int, error) {
+	return 0, io.EOF
+}
+func (r *errorSecondSeekableReader) Seek(offset int64, whence int) (int64, error) {
+	r.count++
+	if r.count == 2 {
+		return 0, r.err
+	}
+	return 0, nil
+}
diff --git a/transport/http/request.go b/transport/http/request.go
index cb8a3c3e1..755e94bcb 100644
--- a/transport/http/request.go
+++ b/transport/http/request.go
@@ -38,6 +38,32 @@ func (r *Request) Clone() *Request {
 	return &rc
 }
 
+// StreamLength returns the number of bytes of the serialized stream attached
+// to the request and ok set. If the length cannot be determined, an error will
+// be returned.
+func (r *Request) StreamLength() (size int64, ok bool, err error) {
+	if l, ok := r.stream.(interface{ Len() int }); ok {
+		return int64(l.Len()), true, nil
+	}
+
+	if !r.isStreamSeekable {
+		return 0, false, nil
+	}
+
+	s := r.stream.(io.Seeker)
+	endOffset, err := s.Seek(0, io.SeekEnd)
+	if err != nil {
+		return 0, false, err
+	}
+
+	_, err = s.Seek(r.streamStartPos, io.SeekStart)
+	if err != nil {
+		return 0, false, err
+	}
+
+	return endOffset - r.streamStartPos, true, nil
+}
+
 // RewindStream will rewind the io.Reader to the relative start position if it
 // is an io.Seeker.
 func (r *Request) RewindStream() error {