Tidal stream energy projects are not likely to materialise unless they are proven to be economic. In this work, we present a new geospatial tool to determine the optimum sites for tidal stream energy exploitation based on this fundamental requirement of economic feasibility, and more specifically, on the LCOE (levelised cost of energy): LCOE mapping. The methodology combines numerical model results of the resource with economic assessments in a Matlab-based tool designed ad hoc. Thus, not only the spatial variability of the resource but also its all-important temporal variability is considered. In particular, the energy density is obtained by means of numerical integration of the power density throughout a tidal cycle. The new tool allows for considering the diameter and power curve of the specific turbines, too. As a case study, the method is applied to the Bristol Channel and Severn Estuary (UK), one of the most promising areas for tidal stream exploitation in the UK. Additionally, a sensitivity analysis is performed and thus, cost trends are analysed. In summary, the LCOE map constitutes a new tool to assess tidal stream sites in an area of interest based on a fundamental economic parameter of an energy project: the LCOE.