ZenithOptimedia will now assume control of all Aviva's global media planning, buying and digital activities, previously split between numerous agencies.
The agency fought off competition from Aegis, Omnicom and WPP agency networks to handle a new account that is expected to make "significant costs savings" across 28 countries.
The move follows much of the UK business rebranding from Norwich Union to Aviva last June. The new business win will include the media requirements for the Aviva-owned motoring company RAC.
At present, Omnicom handles Aviva's £50m media business in the UK, the company's biggest advertising market, while Aegis handles it in Ireland and WPP controls its French business.
Amanda Mackenzie, chief marketing officer at Aviva, said: "We've created a single global brand - a key step in our business transformation - and now is the time to move to a global media agency. An integrated, strategic approach to media will amplify our brand around the world.
"Alongside their first-rate media buying capabilities and international reach, we've been impressed by ZenithOptimedia's grasp of our brand promise and we're looking forward to building a global partnership with them."
There are no plans to review the group's creative requirements, the bulk of which is handled by Abbott Mead Vickers BBDO in a number of markets, including the UK.