Leg won the business after a three-way pitch against Karmarama and TBWA\London.
55DSL wants to raise its profile and distinguish itself from the main Diesel brand. Leg is believed to be developing a visual campaign around the idea "feel at home".
Kristian Kristiansen, the head of marketing at 55DSL, first approached agencies in May, after the company revealed it would be reviewing out of the Amsterdam-based agency KesselsKramer.
55DSL launched in 1994 and is distributed globally. The brand is available in the UK, Japan, Italy, Germany, France and the US. The clothing range is designed to appeal to young consumers who are interested in action sports and urban culture.
Diesel's main fashion brand handed its pan-European account to the Havas-owned ER27 in April.