The Thai national was most recently managing director of McCann Worldgroup in Indonesia and lead Universal McCann in Thailand prior to this appointment.
Chairat brings with him over 20 years' experience in the business and has had spells at Leo Burnett, Saatchi & Saatchi and Grey in Bangkok too.
On the appointment, Publicis APAC business director and Thailand CEO Mark Ingrouille said that he was "thrilled" to be working with Chairat again.
He added: "His energy, enthusiasm, and sometimes his stubbornness, is exactly what we want to help us with our ambitious growth drive in the Kingdom. I've been lucky enough to work alongside Somyot before and when I found out that he'd become available, we wasted no time in getting him on board."