Armistead will join the agency at the end of October from Grey Group Spain, where he was chief strategy officer across the Madrid and Barcelona offices. He will report to Helen Andrews, the managing director of W&K London.
He replaces Paul Colman, who left in January after a decade with the agency following claims of harassment.
Armistead said: "I can honestly say there's only one challenge that could’ve tempted me back onto the London Underground. W&K's independence, unflinching commitment to creativity, and the courage of the clients it attracts make it a unique place. In every sense, I am delighted to be coming home."
Prior to Grey, Armistead was head of planning at 180 Amsterdam, global planning director at W&K Amsterdam and senior planner at Dare. He has worked with brands including Nike, Coca-Cola, EA Sports, Heineken and the Metropolitan Police.
Andrews added: "We are super excited to be welcoming Ben to the management team. We believe his experience will prove invaluable as we continue to steer the good ship W&K London into exciting new places, and it’s clear Ben is also a thoroughly lovely person to have on-board as we do so."