When the ranges of two species overlap and the species can hybridize, some individuals may waste gametes on inviable or infertile hybrids. In these cases, enhanced reproductive isolation may evolve as a byproduct of selection against maladaptive hybridization in a process called reinforcement. On the slopes of the African island of São Tomé, Drosophila yakuba and its endemic sister species D. santomea have a well-demarcated hybrid zone. D. yakuba females from within this zone, but not from outside it, show an increase in gametic isolation from males of D. santomea. To understand why reinforced gametic isolation does not spread to the whole geographic range and stays confined to areas of secondary contact, we studied the associated costs of reinforced gametic isolation in D. yakuba by using a combination of natural collections and experimental evolution. We found that D. yakuba males from sympatric populations sire fewer progeny than allopatric males when the female involved in the mating is an allopatric female. The results here shown suggest that the advantageous nature of reinforcement in D. yakuba is local, as its associated costs (i.e., reduced male fertility) might prevent its dispersal outside the hybrid zone.