Instagram added private messaging to its app in 2013.
It's unclear whether Direct will fully replace the Instagram inbox. Users who download Direct will see their Instagram inbox disappear, but it comes back when the messaging-only app is deleted.
Instagram's latest move follows parent company Facebook's decision to remove private chats from its main app. In 2014, Facebook forced users to download Messenger in order to privately message Facebook friends on mobile -- an effort that was widely criticized. But the app now has over one billion monthly active users.
It's likely Instagram will receive some of the same backlash -- users may not want to have to download yet another app.
Facebook continues to expand its mobile footprint with messaging apps. The company recently launched a private chat app for children ages 6 to 12.