Neoantigens arise from mutations in cancer cells and are important targets of T cell-mediated anti-tumor immunity. Here, we report the first open-label, phase Ib clinical trial of a personalized neoantigen-based vaccine, NEO-PV-01, in combination with PD-1 blockade in patients with advanced melanoma, non-small cell lung cancer, or bladder cancer. This analysis of 82 patients demonstrated that the regimen was safe, with no treatment-related serious adverse events observed. De novo neoantigen-specific CD4+ and CD8+ T cell responses were observed post-vaccination in all of the patients. The vaccine-induced T cells had a cytotoxic phenotype and were capable of trafficking to the tumor and mediating cell killing. In addition, epitope spread to neoantigens not included in the vaccine was detected post-vaccination. These data support the safety and immunogenicity of this regimen in patients with advanced solid tumors (Clinicaltrials.gov: NCT02897765). Display omitted •The personalized neoantigen vaccine Neo-PV-01 plus nivolumab is feasible and safe•NEO-PV-01 plus nivolumab stimulates durable neoantigen-specific T cell reactivity•NEO-PV-01-specific T cells have cytotoxic potential and can traffic to the tumor•NEO-PV-01 induces epitope spreading consistent with vaccine-mediated tumor cytotoxicity In a phase Ib clinical trial, Ott et al. demonstrate feasibility, safety, and immunogenicity of the combination of personalized neoantigen vaccines and PD-1 inhibition in patients with advanced solid tumors. Vaccine-induced T cells persist over time, exhibit cytotoxic potential, and can migrate to tumors. Epitope spread and major pathologic tumor responses were detected following vaccination.