St Luke’s won the account after a pitch against the incumbent, Contagious Content, and will begin work immediately. The agency will create an ad campaign for spring/summer 2014.
St Luke’s already holds the creative business for two other Shop Direct brands, Very and Littlewoods, which it won in 2012.
The TV personality Carol Vorderman is the brand ambassador for Isme, which is aimed at women over 50.
Shop Direct spent £12.2 million on media for Isme in 2013, according to Nielsen.
Jon Owen, the retail brand director at Shop Direct, said St Luke’s won the pitch because of "audience insight" and "creative work that both builds the brand and works hard commercially".
In a separate move, Shop Direct has moved its media planning business from Carat to Vizeum.