Background: Melanoma has a response rate of 10-15% with anti-PD-1 re-challenge in the refractory setting. Newer targeted therapies in melanoma are needed, especially once patients progress on immune checkpoint inhibitor (ICI) therapy. Analysis of TCGA and the Cleveland Clinic’s Gross Family Melanoma Registry reveals that a significant proportion (~40%) of melanoma patients possess somatic (31.6%) or germline (TCGA: 4.2%; Registry: 8.3%) mutations in homologous recombination repair genes, which may serve as a therapeutic target. PARP inhibitors, specifically talazoparib, have demonstrated synthetic lethality, potent PARP trapping activity, and increased immunogenicity of tumor cells by promoting T cell and NK cell infiltration in vitro and in vivo. Moreover, augmentation of the STING pathway via PARP inhibition modulates the tumor microenvironment, impacting PD-L1 expression and type I interferon production. Therefore, the use of talazoparib in combination with the ICI, nivolumab, may have a synergistic immunomodulatory and antitumor effect. Methods: This phase II, single arm, open label trial aims to enroll 37 primary or recurrent unresectable or metastatic melanoma patients harboring a somatic or germline mutation or deletion in BRCA or BRCAness (genes including ARID1A/B/2, ATM, ATR, BAP1, BARD1, BLM, BRCA1/2, BRIP1, CDK4/12, CHEK1/2, DSS1, EMSY, ERCC3, FANCA/D2, HDAC2, IDH1, LIG3/4, MDC1, MLH1/3, MRE11, NBN, PALB2, PRKDC, RAD50/51/54, XRCC6) who have progressed on prior ICI therapy. Patients will be treated with nivolumab 480mg IV every 4 weeks and talazoparib 1mg PO daily. The primary objective is to determine clinical efficacy of the combination therapy, as measured by the objective response rate. The trial is designed to test the null hypothesis that ORR ≤ 10% and is powered to detect an effect size of ORR ≥ 30%. Secondary objectives include PFS, OS, immune-related objective response rate (irORR), irPFS, and treatment-related adverse events. Associations with clinical response will be assessed with correlative studies of PD-L1 expression, ctDNA, tumor mutational burden, copy number variation, and the phenotypic and functional characterization of circulating and tumor infiltrating immune cells. The study is currently open and enrolling patients. Clinical trial information: NCT04187833