Despite the World Health Organization considering it important to promote physical activity as part of a healthy lifestyle, the official data show an increase in the percentage of physical inactivity, which has brought about the development of strategies at different levels (national and international) to reverse this trend. For the development of these strategies, it is relevant to know what the determinants of physical activity (at leisure and at work) are. Therefore, this is going to be analysed in the autochthonous young adults from Seville. A cross-sectional survey of their health behaviours was carried out. The sample was selected through a proportionally stratified random sampling procedure. From the results, we highlight that the general perceived health status is good and that most physical activity is performed during leisure time. However, a majority of the population analysed reported overweight or obesity. Participants with a low perceived health status, those who have low social support from their family and friends, and those who do not smoke are the ones who have more probability of engaging in physical activity during their leisure time. However, gender, education level, and alcohol consumption are revealed as determinants of the intensity of physical activity at work. In this regard, men and/or participants with a low level of studies are those who carry out more physically demanding activities at work.