Lexis won the brief after a five-way shoot out (revealed in PRWeek, 13 January).
The win comes after the consumer agency's acquisition of social and media marketing agency Paratus Communications last week.
Pizza Hut head of marketing Chris Tebben said: 'Pizza Hut UK is taking huge strides to create an exciting, memorable guest experience, and we look forward to partnering with Lexis to ensure that the whole of the UK knows about these exciting changes.'
Lexis will be responsible for running a proactive consumer press office throughout the year and delivering projects focused around specific product and service innovations.
Lexis CEO Jason Gallucci said: 'We look forward to applying our unique approach of generating recommendations through key influencers to help this much-loved family brand meet both its communications and commercial objectives in the short and long term.'
It is understood that Pizza Hut is also seeking agency support for the corporate side of its business, for which incumbent Freud Communications has pitched.
Freuds formerly handled consumer PR as part of an integrated brief. The agency declined to pitch for the newly separated consumer account.
According to market research firm Mintel's 2011 report on pizza and pasta restaurants, Pizza Hut is continuing to lead the pizza outlet market but its closest rival, Domino's Pizza, is rapidly closing the gap.
The Pizza Hut chain, which has more than 700 outlets in the UK, is being squeezed by the dual threat of delivery companies with low overheads, such as Domino's Pizza, and more upmarket offerings such as Pizza Express.