The opening of a new establishment is a critical factor for firms in the retail sector because the decision carries with it a series of very serious financial and corporate image risks. This paper presents the development of a methodology for the process of selecting a retail site location that combines geographic information systems (GIS) and the analytical hierarchy process (AHP). The AHP methodology shows that the success factors for a supermarket are related to its location and competition. The proposed retail site location decision process was applied to the opening of a new supermarket in the Spanish city of Murcia.