Physical activity is a key element in people’s health as it provides important physical benefits, as well as improves mental health and quality of life. However, recent years have seen an increase in the percentage of young adults showing high levels of inactivity. Although, it has been observed that the motivation to perform physical activity seems to be an important factor when starting and then keeping it up. Thus, the general aim of this work was to explore the association between physical activity, motivation, and psychological well-being in young adults. To do this, a descriptive cross-sectional correlational study was carried out together with a multiple linear regression analysis. An online survey was applied between December 2017 and the first quarter of 2018, in which the level of activity, motivation, and psychological well-being of the participants were measured. Starting from a final sample of 489 subjects aged between 18 and 35 years, a higher level of physical activity was found to be related to higher psychological well-being. In addition, motivation, and more specifically intrinsic motivation, was an important determinant of psychological well-being, gaining greater influence among male participants who had a higher level of physical activity. This study therefore emphasizes the clear influence of physical activity on the psychological well-being of young adults and highlights the need to work on intrinsic motivation to improve levels of physical activity.