diff --git a/api-reports/2_12.txt b/api-reports/2_12.txt
index 0c7e86bb3..0d7e7afd5 100644
--- a/api-reports/2_12.txt
+++ b/api-reports/2_12.txt
@@ -16542,9 +16542,12 @@ RequestDestination[SO] val sharedworker: RequestDestination
 RequestDestination[SO] val subresource: RequestDestination
 RequestDestination[SO] val unknown: RequestDestination
 RequestDestination[SO] val worker: RequestDestination
+RequestDuplex[JT]
+RequestDuplex[SO] val half: RequestDuplex
 RequestInit[JT] var body: js.UndefOr[BodyInit]
 RequestInit[JT] var cache: js.UndefOr[RequestCache]
 RequestInit[JT] var credentials: js.UndefOr[RequestCredentials]
+RequestInit[JT] var duplex: js.UndefOr[RequestDuplex]
 RequestInit[JT] var headers: js.UndefOr[HeadersInit]
 RequestInit[JT] var integrity: js.UndefOr[String]
 RequestInit[JT] var keepalive: js.UndefOr[Boolean]
diff --git a/api-reports/2_13.txt b/api-reports/2_13.txt
index 0c7e86bb3..0d7e7afd5 100644
--- a/api-reports/2_13.txt
+++ b/api-reports/2_13.txt
@@ -16542,9 +16542,12 @@ RequestDestination[SO] val sharedworker: RequestDestination
 RequestDestination[SO] val subresource: RequestDestination
 RequestDestination[SO] val unknown: RequestDestination
 RequestDestination[SO] val worker: RequestDestination
+RequestDuplex[JT]
+RequestDuplex[SO] val half: RequestDuplex
 RequestInit[JT] var body: js.UndefOr[BodyInit]
 RequestInit[JT] var cache: js.UndefOr[RequestCache]
 RequestInit[JT] var credentials: js.UndefOr[RequestCredentials]
+RequestInit[JT] var duplex: js.UndefOr[RequestDuplex]
 RequestInit[JT] var headers: js.UndefOr[HeadersInit]
 RequestInit[JT] var integrity: js.UndefOr[String]
 RequestInit[JT] var keepalive: js.UndefOr[Boolean]
diff --git a/dom/src/main/scala-2/org/scalajs/dom/RequestDuplex.scala b/dom/src/main/scala-2/org/scalajs/dom/RequestDuplex.scala
new file mode 100644
index 000000000..de0973d2c
--- /dev/null
+++ b/dom/src/main/scala-2/org/scalajs/dom/RequestDuplex.scala
@@ -0,0 +1,13 @@
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/**
+  * Fetch APIs [[https://fetch.spec.whatwg.org/#dom-requestinit-duplex RequestDuplex enum]]
+  */
+@js.native
+sealed trait RequestDuplex extends js.Any
+
+object RequestDuplex {
+  val half: RequestDuplex = "half".asInstanceOf[RequestDuplex]
+}
diff --git a/dom/src/main/scala-3/org/scalajs/dom/RequestDuplex.scala b/dom/src/main/scala-3/org/scalajs/dom/RequestDuplex.scala
new file mode 100644
index 000000000..41f5092ba
--- /dev/null
+++ b/dom/src/main/scala-3/org/scalajs/dom/RequestDuplex.scala
@@ -0,0 +1,12 @@
+package org.scalajs.dom
+
+import scala.scalajs.js
+
+/**
+  * Fetch APIs [[https://fetch.spec.whatwg.org/#dom-requestinit-duplex RequestDuplex enum]]
+  */
+opaque type RequestDuplex <: String = String
+
+object RequestDuplex {
+  val half: RequestDuplex = "half"
+}
diff --git a/dom/src/main/scala/org/scalajs/dom/RequestInit.scala b/dom/src/main/scala/org/scalajs/dom/RequestInit.scala
index f43401815..de8ea9bab 100644
--- a/dom/src/main/scala/org/scalajs/dom/RequestInit.scala
+++ b/dom/src/main/scala/org/scalajs/dom/RequestInit.scala
@@ -30,6 +30,11 @@ trait RequestInit extends js.Object {
 
   var signal: js.UndefOr[AbortSignal] = js.undefined
 
+  /** "half" is the only valid value and it is for initiating a half-duplex fetch (i.e., the user agent sends the entire
+    * request before processing the response).
+    */
+  var duplex: js.UndefOr[RequestDuplex] = js.undefined
+
   /** The whatwg spec section on [[https://fetch.spec.whatwg.org/#requestinit RequestInit dictionary]] has a comment
     * that states that this value "can only be set to null". In the detailed steps section for
     * [[https://fetch.spec.whatwg.org/#dom-request the Request(input,init) constructor]] it says even more clearly: "If