diff --git a/actix-http/CHANGES.md b/actix-http/CHANGES.md
index 823633753..9bbddd712 100644
--- a/actix-http/CHANGES.md
+++ b/actix-http/CHANGES.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Update `TestRequest::set_payload` to generate "Content-Length" header
 - Malformed websocket frames are now gracefully rejected.
 
 ## 3.11.0
diff --git a/actix-http/src/test.rs b/actix-http/src/test.rs
index dfa9a86c9..5632a310c 100644
--- a/actix-http/src/test.rs
+++ b/actix-http/src/test.rs
@@ -11,7 +11,7 @@ use std::{
 
 use actix_codec::{AsyncRead, AsyncWrite, ReadBuf};
 use bytes::{Bytes, BytesMut};
-use http::{Method, Uri, Version};
+use http::{header, Method, Uri, Version};
 
 use crate::{
     header::{HeaderMap, TryIntoHeaderPair},
@@ -98,9 +98,13 @@ impl TestRequest {
     }
 
     /// Set request payload.
+    ///
+    /// This sets the `Content-Length` header with the size of `data`.
     pub fn set_payload(&mut self, data: impl Into<Bytes>) -> &mut Self {
         let mut payload = crate::h1::Payload::empty();
-        payload.unread_data(data.into());
+        let bytes = data.into();
+        self.insert_header((header::CONTENT_LENGTH, bytes.len()));
+        payload.unread_data(bytes);
         parts(&mut self.0).payload = Some(payload.into());
         self
     }
diff --git a/actix-web/src/types/json.rs b/actix-web/src/types/json.rs
index 22ed624c3..58464f360 100644
--- a/actix-web/src/types/json.rs
+++ b/actix-web/src/types/json.rs
@@ -616,7 +616,7 @@ mod tests {
             }
         ));
 
-        let (req, mut pl) = TestRequest::default()
+        let (mut req, mut pl) = TestRequest::default()
             .insert_header((
                 header::CONTENT_TYPE,
                 header::HeaderValue::from_static("application/json"),
@@ -624,6 +624,7 @@ mod tests {
             .set_payload(Bytes::from_static(&[0u8; 1000]))
             .to_http_parts();
 
+        req.head_mut().headers_mut().remove(header::CONTENT_LENGTH);
         let json = JsonBody::<MyObject>::new(&req, &mut pl, None, true)
             .limit(100)
             .await;