Direct messages sent on Twitter will be end-to-end encrypted - this means private messages can only be read by the sender and the recipient.
Chief executive Elon Musk said it meant he could not see users' messages "even if there was a gun to my head".
But he warned that this is an early version, and people should "try it, but don't trust it yet".
The feature is not yet open to everyone, and can only be used by people who pay for Twitter Blue or are affiliated to a verified Twitter account.
It is also not yet possible to send media in the messages - users can only send text and links.
In a post on its support site, Twitter said it was "not quite there yet" with encryption.
"While messages themselves are encrypted, metadata (recipient, creation time, etc) are not, and neither is any linked content," it said.
"If someone - for example, a malicious insider, or Twitter itself as a result of a compulsory legal process - were to compromise an encrypted conversation, neither the sender or receiver would know."