Background: Mental health problems are very common among medical students, becoming a concern for health care professionals and educators. Despite the fact that there is a growing body of literature supporting the role of religiosity and spirituality (RS) on mental health and on positive psychology outcomes, little evidence is available among medical students, a population subjected to important distress. Aims: This study aimed to investigate the role of RS on the levels of optimism, pessimism, and happiness among Brazilian medical students. Methods: This is a cross-sectional study including medical students from the first to the eighth semester of a private medical school. The instruments used were: TOT-R for optimism and pessimism, Lyubomirsk and Lepper subjective happiness scale, and sociodemographic profile. The independent variables were Brazilian Portuguese Duke University Religiosity Scale for religiosity, Brief Spiritual Religious Coping (SRC) for negative and positive SRC; and Facit-Sp for spiritual well-being. Results: A total of 353 medical students were included (response rate 97.51%). Higher levels of intrinsic religiosity and peace were associated with greater happiness and optimism. However, negative SRC was associated with lower levels of happiness and optimism and higher levels of pessimism. Conclusion: Medical students tend to use their RS as a resource for dealing with the challenges of medical school, influencing their happiness and optimism. In this context, it seems that religiousness acts in two different ways, not only have positive but also negative outcomes among the participants.