Croot, who had worked on the Barclays account for Brunswick for the past five years, replaces Alistair Smith, who became head of corporate comms for Barclays Corporate in August.
Croot has spent more than a decade at Brunswick, working on corp comms for clients including ITN, Britvic and Logica.
He joined the agency in 2000 after a career in broadcast journalism. He was news editor at ITN and has also worked at GMTV, Sky News and the BBC.
He starts at Barclays on 8 November and will work alongside former Cabinet Office comms head Howell James, now corporate affairs director at the bank.