African American (AA) families have the highest risk of prostate cancer (PC). However, the genetic defects contributing to PC in these families remain poorly understood. We performed Whole-exome sequencing of one affected and one unaffected brother in a large AA family with hereditary PC. The novel non-synonymous variants discovered only in the affected were analyzed in all affected and unaffected men in twenty AA-PC families. Here, we report one rare recurrent ADPRHL1 germline mutation (c.A233T; p.D78V) in four of the twenty families. The mutation co-segregates with PC in two families and presents in two affected men in other two, but was absent in 170 unrelated healthy AA men. The mutation expressed in benign prostate cells showed aberrant promotion of cell proliferation while expression of the wild-type ADPRHL1 in PC cells suppressed cell proliferation and oncogenesis. Mechanistically, the ADPRHL1 mutant activates PARP1 leading to an increased H2O2 or cisplatin induced DNA damage response for PC cell survival. Indeed, the PARP1 inhibitor, Olaparib, suppresses PC cell survival induced by mutant ADPRHL1. Given that the expression levels of ADPRHL1 are significantly high in normal prostate tissues and reduce as Gleason scores increase in tumors, our findings provide genetic, biochemical, and clinic-pathological evidence that ADPRHL1 is a tumor suppressor in prostate. A loss of function mutation in ADPRHL1 induces prostate tumorigenesis and confers PC susceptibility in high-risk AA families. Implications: This study highlighted a potential strategy of ADPRHL1 mutation detection in PC risk assessment and a potential therapeutic application for those within AA-PC families.