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A recent first-in-human clinical trial demonstrated that survival in glioblastoma (GBM) patients following rQNestin34.5v.2 oncolytic virus treatment was associated with immune activation signatures. This study was registered at ClinicalTrials.gov (NCT03152318). Here, we provide in situ evidence of ongoing T cell-mediated cytotoxicity against tumor cells at late time points following single treatment, with deep and persistent T cell infiltration into tumor regions. Shorter distances between cleaved caspase-3 tumor cells and granzyme B T cells were associated with longer progression-free survival following treatment. Pre-existing tumor-infiltrating T cells expanded locally upon treatment, correlating with longer overall patient survival. T cells with an early activation program closely interacted with tumor cells and were strongly enriched upon treatment. Viral remnants were restricted to necrotic regions, while T cells infiltrated deeply into live tumor regions. These data demonstrate that single oncolytic virus treatment can expand pre-existing T cell clones and trigger persistent T cell-mediated immunity against GBM.