Poly (ADP-ribose) polymerase inhibitors (PARPis) represent a promising new class of agents that have demonstrated efficacy in treating various cancers, particularly those with BRCA1/2 mutations. Cancer-associated BRCA1/2 mutations disrupt DNA double-strand break (DSB) repair via homologous recombination (HR). PARP inhibitors (PARPis) have been used to trigger synthetic lethality in BRCA1/2-mutated cancer cells by promoting the accumulation of toxic DSBs. Unfortunately, resistance to PARPis is common and can occur through multiple mechanisms, including the restoration of HR and/or stabilization of replication forks. To gain a better understanding of the mechanisms underlying PARPis resistance, we conducted an unbiased CRISPR-pooled genome-wide library screen to identify new genes whose deficiency confers resistance to the PARPi olaparib. Our research revealed that haploinsufficiency of the ZNF251 gene, which encodes zinc finger protein 251, is associated with resistance to PARPis in various breast and ovarian cancer cell lines carrying BRCA1 mutations. Mechanistically, we discovered that ZNF251 haploinsufficiency leads to stimulation of RAD51-mediated HR repair of DSBs in olaparib-treated BRCA1-mutated cancer cells. Moreover, we demonstrated that a RAD51 inhibitor reversed PARPi resistance in ZNF251 haploinsufficient cancer cells harboring BRCA1 mutations. Our findings provide important insights into the mechanisms underlying PARPis resistance by highlighting the role of RAD51 in this phenomenon.