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.
Mr Musk, who has changed the name of the business to X Corp, said the replacement “should have been done a long time ago”.
He posted an image of a flickering X on Twitter, and later in a Twitter Spaces audio chat, replied “Yes” when asked if the Twitter logo would change.
Linda Yaccarino, Twitter’s new chief executive, wrote on the platform that the rebrand was an exciting new opportunity.
“Twitter made one massive impression and changed the way we communicate,” she said.
“Now, X will go further, transforming the global town square.”
The bird is called Larry which Twitter’s co-founder Biz Stone said, in 2011, is a tribute to basketball star and Boston Celtics legend Larry Bird.
People took to Twitter to mourn the loss of the logo, including Martin Grasser who designed it in 2012.
“Today we say goodbye to this great blue bird,” he said. Later the tweet was shared by Jack Dorsey, Twitter’s best-known co-founder with a goat emoji, which means ‘greatest of all time’.
The everything app
For some people in Asia, super-apps including India’s PayTM and Indonesia’s GoJek have been a vital part of everyday lives for the past few years.
The apps let users pay for services through a finance system.
WeChat is a messaging and social media platform that has evolved into one of the region’s biggest apps in terms of its range of services and number of users.
Last year, it was estimated to have 1.29 billion users in China alone.
Drew Benvie, social media commentator and founder of digital agency, Battenhall said, “Musk is going full throttle for the everything app space, leaving old Twitter in his wake, and eyeing the successes of Asia’s trailblazers like WeChat and Moj.
“Succeeding in just a couple of additional services, such as shopping or payments, could be all that’s needed to make X better than Twitter was. But there are already so many alternatives, so Musk and co are playing an enormous game of catch-up.”
Twitter’s website says its logo, depicting a blue bird, is “our most recognisable asset”.
“That’s why we’re so protective of it,” it added.
The firm temporarily replaced the logo in April with Dogecoin’s Shiba Inu dog, helping drive a surge in the meme coin’s market value.
Mr Musk was later accused of insider trading by a group of Dogecoin investors, who claimed he had profited from driving up Dogecoin’s value.
Rapid rebrand
Business commentator Justin Urquhart Stewart said Twitter’s “loyal but aging base” would not like the changes.
“The younger generations have moved onto other apps and Twitter does look at bit old-fashioned.”
“Elon Musk has got to be careful as you are almost starting from scratch with an older audience meanwhile damaging the original brand,” he said.
The very rapid rebranding of Twitter has also caused some security concerns.
Jake Moore, global cyber adviser at security firm ESET, said the transition from one company name to another could encourage phishing, where criminals impersonate people or organisations to steal users’ data.
“A rebrand is the perfect opportunity to send phishing emails requesting users to sign in via a new URL from a link within that email – but of course that link wouldn’t be genuine and that’s where people could be tricked into handing over their genuine Twitter credentials without their usual level of caution,” he told the BBC.
“Cyber-criminals can easily prey on this, especially those searching for that new URL.”
X obsession
Mr Musk has long had a fascination with the letter X – although no-one really knows why.
One of his first business ventures in 1999 was known as X.com, which was an online banking platform.
Just three years later, Mr Musk made $165m when X.com – by then merged with PayPal – was bought by eBay.
He also owns the X.com domain name, which now redirects to Twitter.
Mr Musk is also chief executive of SpaceX, the commercial American aerospace company founded in 2002.
There is also the name of his first child with musician Grimes – X Æ A-12 Musk.
Mr Musk also recently launched his long-awaited artificial intelligence start-up – xAI – in a bid to build an alternative to ChatGPT.