The Choice Experiment (CE) technique is applied for the first time to one of the most promising marine renewables, tidal stream energy, with two objectives: (i) to investigate the public perceptions of this renewable, and (ii) to estimate the externalities, i.e., the monetary value of the impacts of a tidal stream farm. Both aspects, public perceptions and externalities, are relevant to the policy makers: if a policy is to maximise social welfare, it should be in line with public supporting attitudes and have positive externalities; moreover, the externalities are a prerequisite for establishing the appropriate level of subsidy (e.g., through a feed-in tariff). In this work the environmental and socioeconomic externalities are calculated independently, in a procedure that is illustrated through a case study: a prospective tidal farm in Ria de Ribadeo, an estuary in NW Spain. The public perceptions are found to be generally positive; notwithstanding, a certain degree of NIMBYism is detected. As regards the externalities, we find a positive net value. These findings are encouraging for the development of tidal stream energy, and the quantitative results provide a basis to establish the level of subsidisation.