The agency won the digital account after a pitch process that began in July.
LBi, which is understood to be the supermarket's first dedicated digital agency, will work across the Asda.com, Asda Direct and Asda Financial Services brands as the company looks to increase its online sales.
Asda sells electrical items, furniture, toys and clothing, as well as food and financial products, through the three websites.
Its relationship with Saatchi & Saatchi is unaffected by the appointment.
Asda, which has an "everyday low prices" marketing proposition, is currently engaged in a price war with its supermarket rivals. Last week, Tesco launched its "big price drop" campaign in an effort to win customers from other retailers.
LBi declined to comment on its appointment.