The agency beat Bartle Bogle Hegarty, Joint London and Rapier in a pitch process run through Oystercatchers.
VCCP was the incumbent.
Grey has been asked to define the role of Which? in modern culture and unite the organisation’s products and services under one collective brand idea.
The independent consumer group and charity will now place an increased marketing focus on campaigning and influencing on key consumer issues.
As part of its brief, Grey will also place a significant focus on experience planning and design, particularly in relation to Which?’s digital products.
Media planning and buying continues to be handled by MC&C and Brainlabs.
Grey London’s chief executive Leo Rayman said: "In a world where truth is hard to come by and big business tramples on the little guy, Which? has never been in greater demand. Our job will be to give Which? the place it deserves at the heart of popular culture."
Which? was formed in 1957, and is now the largest independent consumer organisation in the UK, with more than 1.5 million members and supporters.
The organisation is a registered charity, and though best known for its consumer product reviews, also provides a range of services including recommending legal and mortgage advisers, trusted traders, universities and where to give birth. It also campaigns on behalf on consumers.
Sarah Threadgould, the group marketing director at Which?, said: "We were impressed by the ambition and breadth of Grey’s thinking throughout the pitch process.
"They went beyond the brief, showcasing a vision for the brand that matches our own and identifying new business models and propositions that will drive future growth."