The agency triumphed after a three-way shoot-out led by TBWA\London against M&C Saatchi and Y&R in a process overseen by AAR.
The review began in January and was co-ordinated from Four Seasons hubs in Toronto, Singapore and Geneva.
TBWA will be responsible for handling the chain's global advertising, working across its global brand, local marketing, CRM and creative design.
The network will also oversee tactical work for local hotels and support for new projects, of which Four Seasons has more than 50 in development. Four Seasons is expected to appoint a media agency to work alongside TBWA, but this has yet to be confirmed.
Four Seasons, which operates 86 hotels in 35 countries, including two in London, does not currently retain a global agency. In the past, it has worked with a variety of local agencies including Mainspring in Australia and Mullen in the US.
At the beginning of the year, Four Seasons invested in a new website for users to create a more personalised online experience.