Determining the drivers that are responsible for the fine-scale distribution of cetacean species is fundamental to understand better how they respond to changes in their environment. We utilized information theoretic approach to carry out a comprehensive investigation of the key environmental and anthropogenic correlates of habitat use and relative density of harbour porpoises. In all, 273 daily boat surveys over a period of 38 months, between April 2014 and November 2017, were spent in the field monitoring 9417 km along the coastal and shelf waters of Northwest Spain. Throughout this period, there were 70 encounters with harbour porpoises and 712 encounters with common bottlenose dolphins. The observed unequal use of available habitat indicates that harbour porpoises present a fine-scale pattern of habitat selection along the study area, which is likely related to the variation in oceanographic variables and human disturbance mainly caused by marine traffic and fisheries. While differences in habitat use between harbour porpoises and bottlenose dolphins were observed, interspecific competition with bottlenose dolphins (as competitive exclusion hypotheses) did not appear to play an important role in the distribution and relative density of harbour porpoises. These findings highlight the importance of considering both environmental and anthropogenic variables in ecological studies, in addition to highlighting the importance of using a multi-species ecology approach in research and conservation management planning.