Responsible Sourcing is gaining growing importance for companies willing to demonstrate responsibility and commitment to sustainable practices. Sourcing is a key element of supply chain management and by adopting responsible sourcing practices, companies can greatly contribute to the sustainability of their entire supply chains. Being close to both customers and suppliers, retailers hold a particularly influential position in the supply chain and their sourcing choices can play a key role for sustainability improvements. This paper investigates the different strategies that can be adopted to guarantee responsible sourcing in the retail sector, through the specific case study of two UK retailers, Sainsbury´s and Tesco, and their offer of sustainable seafood. Specifically, the relation between the offer of MSC certified products and the retailers´ commitment to source responsibly is here analysed. The main source of data collection is a content analysis of retailers´ CSR online reports and web pages. Additional information is obtained through reports and web pages’ analysis of four UK organizations dealing with sustainable seafood issues. The findings of the study suggest that the offer of MSC certified products is positively correlated with the adoption of responsible sourcing practices; different strategies are available to retailers to guarantee responsible sourcing and that retailers´ sourcing policies can be influenced by other stakeholders´ guidelines for responsible sourcing.