University students, as a result of their lifestyles, represent a section of the population that is most likely to adopt sedentary behaviours. The aim of the present study was to analyse the determining factors dictating the performance of physical activity as well as sedentary behaviour among university students. A total of 608 students (64.6% women) from the University of Santiago de Compostela (Spain) were selected by stratified random sampling to take part in the study, which involved completing a questionnaire on lifestyle and physical activity. Of the participating students, 69.6% indicated that they performed physical activity; the main reasons given were to maintain fitness and for health, while a lack of time and laziness were the principal reasons given for abandoning or not taking up physical exercise. Significant associations were established between not doing physical activity and the time exposed to screens, time studying, feeling low and smoking; on the other hand, associations could be seen between doing physical activity and the participation of relatives (parents, mothers, partners, older siblings and friends) in physical activity, as well as a positive sense of satisfaction relating to physical education taught in schools. In conclusion, most of the university students did some physical activity, which was associated with less sedentary behaviour, while the influence of school physical education and of the habits of relatives played an important role.