Astrophytum asterias is an endangered cactus occurring in southern Texas and northeastern Mexico. The purpose of this study was to provide fundamental information about the reproductive biology of the species. Breeding-system experiments were conducted in Starr County, Texas. Pollination treatments were applied to explore whether the species is self-compatible or self-incompatible and whether plants are experiencing pollinator limitation. Treated (bagged) flowers were self-pollinated or outcrossed by hand, or allowed to naturally self-pollinate. Control flowers were unbagged and unmanipulated (open-pollinated). In each treatment and controls, we calculated mean fruit set and mean seed set per plant. Controls had 18.3% fruit set, with a mean seed set of 13.3 ± 14.7 seeds. The outcross treatment resulted in 88.6% fruit set, with a mean seed set of 95.8 ± 5.9 seeds. Median mean fruit and seed set in the outcross treatment were 1 and 92.7, respectively. Although some controls set fruit and seed, median mean fruit and seed set was 0. No fruit or seed was set in self-pollinated treatments. These results showed that the species is an obligate outcrosser. Reproductive success is, therefore, dependent on a vector to transfer pollen. The fact that fruit set and seed set were higher in hand-outcrossed flowers than in open-pollinated flowers suggests that A. asterias might be experiencing reproductive constraints in terms of availability or effectiveness of pollinators.