The new white X on a black background has replaced the blue bird on the desktop version of the social network, although is yet to appear on the mobile app.
“Tweets” will also be replaced, according to Twitter’s owner Elon Musk, and posts will be called “x’s”.
The billionaire changed his profile picture to the new logo and added “X.com” to his Twitter bio.
Mr Musk wants to create a “super app” called X – his vision for a new kind of social media platform that he has been talking about creating for months.
On Sunday, the billionaire said he was looking to change Twitter’s logo, tweeting: “And soon we shall bid adieu to the Twitter brand and, gradually, all the birds.”
He then shared a picture of the new X branding projected onto the side of Twitter headquarters in San Francisco.
