Background: Poly (ADP-ribose) polymerase (PARP) inhibitors mediate significant anticancer effects against homologous recombination defective (HRD) tumors but have limited efficacy against homologous recombination proficient (HRP) neoplasms. Moreover, HRD tumors often become resistant to PARP inhibitors during treatment, constituting a major limitation to the clinical use of these agents. Here, we describe the metabolic, antineoplastic and immunomodulatory effects of a first-in-class oligodeoxynucleotide that operates as a PARP1 decoy (OX413), resulting in constitutive hyperactivation of PARP1 and consequent exhaustion of the DNA damage response. Methods: OX413-induced PARP trapping and hyperactivation, NAD⁺ consumption and cell cytotoxicity were monitored using (HRP and HRD) breast, ovarian or prostate cancer cells as well as normal fibroblasts and leukocytes. DNA repair abrogation was monitored by analyzing repair protein recruitment to damage sites. Accumulation of cytoplasmic DNA fragments was monitored after DNA staining and microscopy analysis. Effects on the innate immune response was assessed by following STING pathway activation and T-cell mediated anti-tumor cytotoxicity. In a mouse xenograft model of breast cancer, OX413-induced PARP activation and tumor infiltration by immune cells were analyzed ex-vivo by flow cytometry. Antitumor efficacy of OX413 alone or combined to a PD1 blocker was assessed in MPA/DMBA-driven HR⁺ HER2^- mouse mammary carcinomas. Results: At odds with conventional PARP inhibitors, OX413 bound to and hyperactivated PARP1 with high affinity and in a dose-dependent manner, resulting in elevated cytotoxicity in multiple cancer (but not normal) cells irrespective of HR status. OX413 impaired DNA repair due to PARP trapping and rapid NAD⁺ consumption, leading to the accumulation of cytoplasmic chromatin fragments (CCFs). Consistent with this, OX413 triggered activation of the CGAS/STING pathway, CCL5 secretion and potentiation of tumor-targeting T cell responses. PARP1 engagement and STING activation were corroborated in vivo in EMT6 mouse mammary carcinomas, correlating with anticancer effects coupled to tumor infiltration by innate and adaptive immune cells. In MPA/DMBA-driven tumors, OX413 mediated considerable anticancer effects that could be exacerbated by PD1 inhibition. Conclusions: Our results provide preclinical rationale for using OX413 to trigger metabolic exhaustion in cancer cells and initiate inflammatory responses that can be actioned by immune checkpoint inhibitors in patients bearing HRD as well as HRP tumors.