Instinct won the six-figure account from Surgery, marking the fashion team's biggest client win to date.
The remit includes handling all consumer and trade media, as well as celebrity outreach across the Replay, Replay One Off, We Are Replay and Replay&Sons collections.
A Replay spokesperson said the two companies were a natural fit.
"Not only is Instinct PR well connected within the fashion PR industry, but we can see the passion and understanding that is necessary when it comes to working with a premium brand such as Replay," the spokesperson said.
"The enthusiasm shown alongside the team’s creative and strategic thinking gives great confidence that the agency will work tirelessly to build on our strong reputation, constantly pushing for further opportunities and better results."
Jonathan Kirkby, founder and director of Instinct PR, said Replay would complement its existing roster of clients, which includes Asos, Bacardi, Puma and Police Watches and Sunglasses.
He said: "We have some very exciting plans in place for the brand over the next six to 12 months and we’re all looking forward to executing them and delivering some exceptional results."