"X is here! Let's do this," tweeted Yaccarino, who also posted a picture of the logo projected on the company's offices in San Francisco.
Both Yaccarino's and Musk's Twitter handles feature the X logo, although the Twitter blue bird is still visible across the platform.
"#GoodbyeTwitter" was trending on the platform with reference to the old logo as several users criticised the new one.
He posted a picture of a stylised X against a black outer space-themed background. He also referred to the "interim X logo," and tweeted that "soon we shall bid adieu to the Twitter brand and, gradually, all the birds".
In response to a tweet asking what will tweets be called under "X", Musk replied "x's". The original Twitter logo was designed in 2012 by a team of three. "The logo was designed to be simple, balanced, and legible at very small sizes, almost like a lowercase "e"," tweeted Martin Grasser, one of the designers.
Matt Rhodes, strategy lead at creative agency House 337, told Reuters any changes to a brand so established in popular culture was a risk.
"Only a few brands have become verbs or seen themselves referred to in global news outlets as often as Twitter has," he said.
"Anything that makes it harder for people to find, or want to open the app on their cluttered phone screens risks harming usage."
Weeks before completing his Twitter acquisition last year Musk had said that buying the company would speed up his ambition to create an "everything app" called "X" by three to five years.
Musk bought x.com back from PayPal in 2017, saying it had "sentimental value". Musk had co-founded x.com as an online bank in 1999 which later transformed into PayPal.
While Twitter's official page on the platform has been renamed as "X", the domain x.com is not active.
"X is the future state of unlimited interactivity - centred in audio, video, messaging, payments/banking - creating a global marketplace for ideas, goods, services, and opportunities," Yaccarino tweeted on Sunday.
Yaccarino, the former advertising chief at NBCUniversal who started as Twitter CEO on June 5, has taken over when the social media platform is trying to reverse a plunge in advertising revenue.
Since the takeover of Twitter, the company has faced tumultuous times with layoffs, a sharp drop in advertisers and the meteoric rise of Threads, Meta's response to Twitter.
"The last few months have been tumultuous at Twitter, and I don't think a new brand is going to solve everything," Drew Benvie, CEO of social media consultancy Battenhall, said.
"This is less about reinventing Twitter, and more about building a brand around Elon Musk's empire, including SpaceX, where the 'X' branding really connects a little more closely."
