diff --git a/build/docs/mozinfo.rst b/build/docs/mozinfo.rst
index e98044100dfc..252a83940a1e 100644
--- a/build/docs/mozinfo.rst
+++ b/build/docs/mozinfo.rst
@@ -136,6 +136,14 @@ release_build
 
    Always defined.
 
+sm_promise
+   Whether spidermonkey promises have been enabled or not.  This is set
+   by adding --enable-sm-promise to the mozconfig file.
+
+   Values are ``true`` and ``false``.
+
+   Always defined.
+
 tests_enabled
    Whether tests are enabled for this build.
 
diff --git a/python/mozbuild/mozbuild/mozinfo.py b/python/mozbuild/mozbuild/mozinfo.py
index cd1868695940..4f40ed7138c6 100755
--- a/python/mozbuild/mozbuild/mozinfo.py
+++ b/python/mozbuild/mozbuild/mozinfo.py
@@ -95,6 +95,7 @@ def build_dict(config, env=os.environ):
     d['addon_signing'] = substs.get('MOZ_ADDON_SIGNING') == '1'
     d['require_signing'] = substs.get('MOZ_REQUIRE_SIGNING') == '1'
     d['official'] = bool(substs.get('MOZILLA_OFFICIAL'))
+    d['sm_promise'] = bool(substs.get('SPIDERMONKEY_PROMISE'))
 
     def guess_platform():
         if d['buildapp'] in ('browser', 'mulet'):