Introduction: Religious factors have conditioned the attitude toward organ donation and transplantation (ODT) since the beginning of transplantation, despite the fact that most religions are in favor of transplantation. Objective: To assess the impact of religious beliefs of medical students on their attitude toward ODT. Method: Population under study: Medical students in Spanish universities. Study sample: Stratified by geographical area and academic course. Assessment instrument: Attitude ODT questionnaire PCID-DTO-Ríos, anonymous and self-administered. Results: Of all students, 42% (n = 3907) declare themselves atheists or agnostics. The remaining 58% (n = 5368) declare themselves to be religious, the majority being Catholic (55%, n = 5102). Of the rest, 0.2% are Muslims (n = 8), 0.1% Protestants (n = 1), and the remaining 2.7% (n = 257) indicate other religious doctrines but do not want to specify it. Regarding their attitude toward ODT, those who consider themselves atheists or agnostics have a more favorable attitude than those who consider themselves religious (84% versus 76%; P < .001). Among those who follow some kind of religion, Catholics are more in favor of ODT than non-Catholics (77% vs 64%, P < .001). Note that among the religious, only 57% (n = 3050) know which religion is in favor of transplantation, while 22% (n = 1,152) consider that it has not been pronounced on the matter, 13% (n = 723) think the religion is against donation, and the remaining 8% (n = 443) do not know. Conclusion: The religion professed by medical students conditions their attitude toward donation, with the atheists and agnostics being more in favor of donation.