Knemidokoptes is a genus of subcutaneous mites found in the skin of multiple avian hosts, although few cases have been reported in wild raptors. Population monitoring of Black Sparrowhawks (Accipiter melanoleucus) on the Cape Peninsula, South Africa, between 2001 and 2012 revealed multiple birds with infection symptoms in some years. Examination of 3 dead birds displaying symptoms such as baldness and skin lesions confirmed infection by Knemidokoptes spp., whereas we found no cases of subclinical infection in birds without symptoms (n = 16). Up to 5% of birds in the population were infected in some years, which represents the first record of multiple birds displaying an infection by Knemidokoptes in a wild population of raptors. A male bias in infection prevalence was detected. Prevalence of infection symptoms was generally low in other populations elsewhere in South Africa, although possibly higher in urban areas. Breeding performance (both productivity and nesting success) was significantly lower for individuals following infection and also in comparison with noninfected birds throughout the study period. This is the first study to demonstrate the negative effect that these mites may have on breeding performance in a wild bird species, and our results suggest that this parasite could potentially influence population dynamics over time.