It is understood that PHD won the business in a pitch that also involved MediaCom.
Bentley, which is owned by the Volkswagen Group, is active in the UK, Asia and the US.
Stefan Buescher, marketing and product director at Bentley said: "We want to strengthen the Bentley brand further in both existing and emerging markets and, by appointing a global media buying and planning agency, we will now be able to fully align all of our media activities around the world.
"This will deliver a powerful, consistent and integrated brand language across both traditional and new media channels."
MediaCom handles the global accounts for VW brands Volkswagen, Seat, Audi, Skoda and Volkswagen Commercial Vehicles.
Two years ago PHD won the estimated £70m global planning and buying business for Porsche, which has close ties with Volkswagen.