Incumbent agency Way To Blue chose not to repitch for the business after mutually agreeing to end its relationship with YO! earlier this year, Way To Blue CEO Adam Rubins told PRWeek.
W said it would help reinvigorate and develop YO! via a mix of new store launches and product storytelling, following the restaurant's rebranding from Yo! Sushi last year.
The agency's remit also includes a series of initiatives designed to bring to life YO!'s "signature innovative and dynamic personality", W said.
W, which started work this month, has also been tasked with supporting YO!'s regional rollout via traditional PR and local influencer programmes.
YO!, which was founded in London in 1997, has roughly 100 restaurants throughout the UK, US, Europe and the Middle East.
W founder Warren Johnson said: "YO! is an iconic brand that has been at the vanguard of the UK's modern dining scene and we are looking forward to working with the brand team to help shape the next stage of its incredible journey."