Possible will continue as a standalone brand within Wunderman. The new group will have approximately 9,200 people within 200 offices across 70 countries.
WPP said that global clients want "much greater simplicity and scale from their digital marketing partners to deal with a more complex, fragmented and changing environment".
Wunderman is a global direct marketing agency. The UK arm picked up the hotly contested BT and EE account earlier this year.
Possible is a digital marketing agency led by global chief executive Shane Atchison.
Mark Read, Wunderman’s global chief executive, said: "I’ve been closely involved with Possible since its inception and have long admired the creative and strategic excellence of its people. Clients today are looking for simplicity and expertise and this move is designed to provide both."
Both networks work with Microsoft, which WPP believes will "benefit from leveraging the strengths" of Possible and Wunderman around the world.
Shane Atchison, global chief executive of Possible, added: "Combining our deep and diverse capabilities is exactly the kind of forward-thinking move that will allow us to deliver business results for our clients."
Last month WPP confirmed plans to merge two Group M media agencies, Maxus and MEC.
Editor's note: This story was updated at 9.20am on 6 July after WPP confirmed news reports that Wunderman and Possible were set to merge.