The pizza brand's incumbent creative shop Ogilvy & Mather London did not repitch for the business, which was awarded to Iris following a four-way contest.
It is understood that the agency has been tasked with defining and launching a fresh proposition for the fast-food delivery chain across TV, digital and customer experience.
This is the second Yum Brands-owned business that Iris has taken on in the past six months, with Pizza Hut's sister brand KFC having appointed the firm to handle its digital and CRM last September.
Pizza Hut's appointment of Iris comes just months after the agency terminated its five-year relationship with the brand's biggest rival Domino's due to a conflict of interest over the KFC account.
When Iris broke ties with Domino's it was only responsible for overseeing the brand's digital functions, with VCCP having snatched away the creative brief months earlier.
Iris is credited with partly influencing Dominos' decision to invest in digital and tech over traditional advertising. It was behind innovations such as the brand's 'Easy Order Button' and 'Pizza Legends' campaign, which it claimed helped drive 38% higher average spend per-order.
Pizza Hut, which had tasked Ogilvy with developing a series of ad campaigns to promote its delivery business, is likely hoping Iris can help turn its brand around.
Following a marketing shakedown last July, the pizza chain's new UK chief sales and brand officer, Stephan Croix, said alongside new sales and marketing director Beverley D’Cruz that the company had to improve the brand's customer experience and digital capabilities.
Commenting on the new appointment, Iris UK managing director Chris Marlow said: "We've had a lot of fun working in this category, achieving 12 quarters of double-digit growth for Domino's along the way.
"This brief from Pizza Hut could not have been more motivating and calls on the forward-thinking disruptive approach that is second nature for Iris. We can't wait to get to work."