The digital agency picked up the account following a pitch against undisclosed agencies to add Wallis to its client list.
Bigmouthmedia runs search marketing for six Arcadia brands including Burton, Dorothy Perkins, Miss Selfridge, Topman and Topshop. Evans and Outfit are the only Arcadia retailers that Bigmouthmedia does not now work with.
The latest win comes after the agency set up a dedicated retail unit working solely with clothing retailers.
Lyndsay Menzies, managing director of Bigmouthmedia UK, said building the unit enabled the agency to boost its sector-specific understanding.
Bigmouthmedia started 2008 by picking up the search brief for Sarah Beeny's dating site, Mysinglefriend.com. Its other clients include British Airways, Barclays and Tesco.