Background: Angiosperms exhibit diverse reproductive systems, and andromonoecy—where individuals produce both hermaphroditic and staminate flowers—is rare, occurring in about 1.7 % of species. The family Zygophyllaceae is common in the Sonoran Desert, yet little is known about the reproductive biology of Guaiacum coulteri, a protected species in Mexico. Preliminary observations suggested the occurrence of two floral morphs, possibly indicating andromonoecy. Questions: What is the breeding system of G. coulteri? Is the species self-compatible or self-incompatible? Studied species: Guaiacum coulteri Study site and dates: Hermosillo, Sonora, 2022 Methods: Floral traits were described, flower longevity and pollinators were documented, and controlled pollination treatments were conducted in a northern population from Hermosillo, Sonora. Results: Two flower morphs were identified: short-gynoecium (SG) and long-gynoecium (LG) flowers. SG flowers did not set fruits, whereas LG flowers produced fruits and lasted longer (two days vs. one). LG flowers exhibited protandry and positive herkogamy. Pollination experiments showed that LG flowers are self-incompatible. Natural and cross-pollination treatments produced very low fruit set (< 4 %), and flowers were mainly visited by native bees. Conclusions: G. coulteri exhibits andromonoecy, with SG flowers functioning as staminate and LG flowers as hermaphroditic. The species is self-incompatible and bee-pollinated. These results represent the first evidence of andromonoecy in G. coulteri and provide key insights into the reproductive ecology of this threatened desert tree.