diff --git a/js/xpconnect/src/Sandbox.cpp b/js/xpconnect/src/Sandbox.cpp
index fc16f076aa96..2a41b6bd0d04 100644
--- a/js/xpconnect/src/Sandbox.cpp
+++ b/js/xpconnect/src/Sandbox.cpp
@@ -1112,6 +1112,7 @@ xpc::CreateSandboxObject(JSContext* cx, MutableHandleValue vp, nsISupports* prin
     priv->allowWaivers = options.allowWaivers;
     priv->isWebExtensionContentScript = options.isWebExtensionContentScript;
     priv->isContentXBLCompartment = options.isContentXBLScope;
+    priv->isSandboxCompartment = true;
 
     // Set up the wantXrays flag, which indicates whether xrays are desired even
     // for same-origin access.
diff --git a/js/xpconnect/src/XPCJSRuntime.cpp b/js/xpconnect/src/XPCJSRuntime.cpp
index 9bf057cc2a7d..327d1840de46 100644
--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -215,6 +215,7 @@ CompartmentPrivate::CompartmentPrivate(JS::Compartment* c)
     , isWebExtensionContentScript(false)
     , allowCPOWs(false)
     , isContentXBLCompartment(false)
+    , isSandboxCompartment(false)
     , universalXPConnectEnabled(false)
     , forcePermissiveCOWs(false)
     , wasNuked(false)
@@ -491,6 +492,16 @@ IsInContentXBLScope(JSObject* obj)
     return IsContentXBLCompartment(js::GetObjectCompartment(obj));
 }
 
+bool
+IsInSandboxCompartment(JSObject* obj)
+{
+    JS::Compartment* comp = js::GetObjectCompartment(obj);
+
+    // We always eagerly create compartment privates for sandbox compartments.
+    CompartmentPrivate* priv = CompartmentPrivate::Get(comp);
+    return priv && priv->isSandboxCompartment;
+}
+
 bool
 IsUniversalXPConnectEnabled(JS::Compartment* compartment)
 {
diff --git a/js/xpconnect/src/xpcprivate.h b/js/xpconnect/src/xpcprivate.h
index 2c646ecdffab..c0aa7727a420 100644
--- a/js/xpconnect/src/xpcprivate.h
+++ b/js/xpconnect/src/xpcprivate.h
@@ -2873,6 +2873,9 @@ public:
     // such a compartment is a content XBL scope.
     bool isContentXBLCompartment;
 
+    // True if this is a sandbox compartment. See xpc::CreateSandboxObject.
+    bool isSandboxCompartment;
+
     // This is only ever set during mochitest runs when enablePrivilege is called.
     // It's intended as a temporary stopgap measure until we can finish ripping out
     // enablePrivilege. Once set, this value is never unset (i.e., it doesn't follow
diff --git a/js/xpconnect/src/xpcpublic.h b/js/xpconnect/src/xpcpublic.h
index 4ff8a6c96ead..e841ab7bdeb8 100644
--- a/js/xpconnect/src/xpcpublic.h
+++ b/js/xpconnect/src/xpcpublic.h
@@ -85,6 +85,8 @@ bool IsContentXBLCompartment(JS::Compartment* compartment);
 bool IsContentXBLScope(JS::Realm* realm);
 bool IsInContentXBLScope(JSObject* obj);
 
+bool IsInSandboxCompartment(JSObject* obj);
+
 // Return a raw XBL scope object corresponding to contentScope, which must
 // be an object whose global is a DOM window.
 //