Rolling out now to select users, the new option aims to better support conversations about sensitive topics.
The chat service already uses "secure communications channels," but "We've heard from you that there are times when you want additional safeguards," Facebook said in an announcement. "Perhaps when discussing private information like an illness or a health issue with trusted friends and family, or sending financial information to an accountant."
So, Facebook is experimenting with one-to-one "secret conversations," which are end-to-end encrypted and can be read only by the person with whom you're communicating, and on only one device.
"That means the messages are intended just for you and the other person—not for anyone else, including us," the blog said.
Starting a secret conversation is entirely optional; users who choose covert communication can set a timer to control how long each message remains visible.
Sharing rich content like GIFs and videos, making payments, an other Messenger features are not currently supported in secret conversations, which are expected to become more widely available this summer.
"We are putting a lot of thought into the design and implementation of this feature, and we are grateful to the security and privacy experts who have given us their valuable input," Facebook said.
The new tool employs Signal Protocol, which also powers creator Open Whisper System's private messaging app Signal, as well as WhatsApp and Google Allo.
Reports about a more secure version of Messenger emerged last month, which said Facebook doesn't want to turn on end-to-end encryption by default because it will mess with its own chatbots.