Eaton, who starts next week in the newly created position, joins having left his role as Barclays MD of corporate comms, corporate and investment banking at the end of last year.
He will report to Tim Fallon, CEO and global managing partner at the London-based agency, who said: "I am delighted that someone of Simon's calibre and experience is joining Instinctif Partners. He is highly regarded within our industry and someone I am very much looking forward to working with."
Eaton has also worked as head of European media relations at Goldman Sachs, along with positions at Burson-Marsteller and Tavistock Communications.
He said: "Instinctif has a strong management team and talented people to allow it to continue to grow and provide excellent UK and international corporate communication services to its clients."
Earlier this year Barclays announced 7,000 job cuts from its investment arm as part of a broader slashing of 14,000 jobs by 2016.