A number of therapies have been approved for patients with metastatic melanoma, but responses are often limited and mortality remains high; therefore, new therapeutic options are needed. In a phase I trial, a novel DNA-repair inhibitor, DT01, was evaluated in 23 patients with melanoma skin metastases. DT01 is an injectable oligonucleotide mimic of DNA lesions that sequesters the DNA-repair machinery from lesions in genomic DNA. Thus, DT01 was used in combination with radiotherapy to induce genomic lesions. Dose escalation revealed no dose-limiting toxicities after intratumoural and peritumoural injection of DT01, and a maximum tolerated dose was not reached; grade 1–2 injection-site reactions were the most common adverse events. Evaluation of efficacy at 76 lesions in 21 patients revealed promising objective and complete response rates of 59% and 30%, respectively.