<p><i>Couroupita guianensis</i> is a native (but not endemic) tree to the Brazilian Amazon rainforest, well known by the pleasant scent of their showy flowers gathered in cauline inflorescences, and their peculiar spherical, heavy, and woody fruits, which makes the species a common component of urban landscapes in Brazil and around the tropical world. Formerly suggested to be xenogamous, this species has been treated as predominantly allogamous but self-compatible since the first criterious study of its breeding system. In this work we revisit the question of self-sterility of <i>C. guianensis</i> in a breeding system study based on controlled pollination experiments and light microscopy analysis of post-pollination events. Only cross-pollinated and open-pollinated flowers produced fruits that reached maturity twelve months later, indicating strong self-sterility. Extended pistil longevity and fruit initiation were observed after selfing, but almost all selfed fruits aborted at the first month after pollination exhibiting significantly greater length and width than unpollinated pistils at the time of abscission. No sign of prezygotic self-incompatibility reactions was verified, and ovule penetration, fertilization, and the nuclear endosperm initiation in selfed pistils/fruits did not differ from the same events in crossed ones up to abscission of the formers. Due to the long resting zygote condition in this species, selfed fruit abscission occurred before normal embryogenesis initiation in crossed seeds, so that there was no embryo formation in selfed seeds. No evidence for early-acting inbreeding depression was found in this study. The data indicate postzygotic late-acting self-incompatibility as the main cause of self-sterility in <i>C. guianensis</i>.</p>