It picked up the business in a pitch against a number of undisclosed rivals. The pitch was handled by the AAR.
Isobel will be briefed to create an overarching campaign that includes above-, below- and through-the-line executions, in order to continue the company's sales growth by building on the brand.
It is the first time Pizza Express has used a single agency of record for its advertising duties. It previously used agencies on an ad-hoc basis.
Neil Wickers, the commercial and marketing director at Pizza Express, said: "Isobel immediately understood who we are, what we're trying to do and provided us with a great creative solution on how to communicate this to customers old and new."
Earlier this month, Pizza Express, which has more than 320 restaurants nationwide, won the Most Revitalised Brand award at the Retailers' Retailer of the Year Awards.
In its September preliminary results, Pizza Express's parent company, the privately owned Gondola Group, posted sales growth of 7 per cent.