The tumor suppressor protein BRCA1 localizes to sites of DNA double-strand breaks (DSB), promoting repair by homologous recombination through the recruitment of DNA damage repair proteins. In normal cells, homologous recombination largely depends on BRCA1. However, assembly of the pivotal homologous recombination regulator RAD51 can occur independently of BRCA1 in the absence of 53BP1, another DNA damage response protein. How this assembly process proceeds is unclear, but important to understand in tumor cell settings where BRCA1 is disabled. Here we report that RNF8 regulates BRCA1-independent homologous recombination in 53BP1-depleted cells. RNF8 depletion suppressed the recruitment of RAD51 to DSB sites without affecting assembly or phosphorylation of the replication protein RPA in neocarzinostatin-treated or X-ray-irradiated BRCA1/53BP1-depleted cells. Furthermore, RNF8/BRCA1/53BP1-depleted cells exhibited less efficient homologous recombination than BRCA1/53BP1-depleted cells. Intriguingly, neither RNF8 nor its relative RNF168 were required for RAD51 assembly at DSB sites in 53BP1-expressing cells. Moreover, RNF8-independent RAD51 assembly was found to be regulated by BRCA1. Together, our findings indicate a tripartite regulation of homologous recombination by RNF8, BRCA1, and 53BP1. In addition, our results predict that RNF8 inhibition may be a useful treatment of BRCA1-mutated/53BP1(low) cancers, which are considered resistant to treatment by PARP1 inhibitors and of marked current clinical interest.