The business moves from Abbott Mead Vickers BBDO, which has held the account, including the high-profile £4m sponsorship of The Simpsons on Channel 4, since 1998. W&K's appointment follows a review of its media and direct business.
Future advertising will build on changes to Pizza Hut's menu last year, which included an extension of its salads range and the introduction of healthier options with a line of lower-fat pizzas branded Hi-Light. The additions matched similar initiatives by Burger King and McDonald's.
The 616-outlet chain, a joint venture between Whitbread and Yum! Brands, plans to open a further 65 restaurants this year.
W&K beat Clemmow Hornby Inge, DDB London and Bartle Bogle Hegarty, which handles the creative account for Yum!-owned KFC. AMV BBDO declined to repitch.
Last month, Pizza Hut handed Elvis its multimillion-pound direct account in a shift of strategic focus away from fast food toward casual dining.
In June, it moved its media business from PHD to Starcom Motive.