According to reports, the Facebook-owned messaging app's TestFlight beta programme includes the setting in some new testing notes.
As MacRumors reports, a beta of the update for the Android version was seen last month, which could hint at the features iOS users can expect in the future.
The update adds a fresh theme option - found in the apps settings - which will let people manually swap between modes or let your phone decide.
Last month, the social media messaging site explained the dark mode feature is "under development" whilst they work out a few kinks.
In a statement, the platform said: "WhatsApp is continuing to work on the Dark Theme, improving the last details before the release.
"It is continuing to add new features for the Dark Theme before the release.
"For this reason, it's normal if you don't see [Dark Theme] in WhatsApp, as it is under development."
The Facebook-owned Instant messaging service made the app's mode to reduce the eye strain and reserve battery power available on Android devices such as Samsung and Huawei on the Beta version of WhatsApp last year.