Background: No effective therapy exists for peritoneal metastases (PM) from gastric cancer (GC), which remain fatal within months of diagnosis. We investigated a novel chimeric oncolytic virus platform, CF33-OV, for its anti-cancer potential against peritoneal cancer cells from GC patients. Methods: We prospectively collected fresh ascites or peritoneal washings from 30 GC patients between 2019 and 2020. These included 16 males (53.3%) and 14 females (46.7%) with a mean age of 51.3 ± 15.1 years. Of the 30 patients, 23 (90%) had cytology or biopsy-proven PM. Cells isolated from the peritoneal fluid were analyzed using flow cytometry for immune cell markers and/or treated with PBS, CF33-GFP (MOI=3), or CF33-hNIS-antiPDL1 (MOI=3) for 15 hours. We evaluated the treated cells for virus infection, replication, CD8⁺ and CD4⁺ expression with exhaustion markers, virus infection, replication, and cytotoxicity. We evaluated ex vivo CD274 (PD-L1) blockade by virus-encoded anti-PD-L1 scFv and the release of growth factors and cytokines following CF33-hNIS-antiPDL1 treatment. Results: The CD45- cancer cell percentage was higher, and the CD8+ T cell percentage was significantly lower in the ascites group than in the peritoneal washing group. At baseline, low levels of CD274 (PD-L1), CD252 (OX40L), and EGFR were identified on the surface of cancer cells. CD4+ and CD8+ T cells did not express cell surface exhaustion markers. After 15 hours of treatment, CF33-GFP infected, replicated in, and killed cancer cells ex vivo with a significantly higher dead cell percentage than the PBS control group (10.8% treated vs. 3.1% control, p-value <0.05). Of the live cancer cells, 69% were GFP+ by flow cytometry. Fluorescence microscopy results confirmed virus-encoded GFP expression in the ex vivo cells. Moreover, CF33-hNIS-antiPDL1 produced functional anti-PD-L1 scFv, which blocked phycoerythrin labeled anti-PD-L1 binding to PD-L1 on live cancer cells. Virus treatment increased CD8⁺CD107α⁺ and CD4⁺CD107α⁺ T cell percentages while decreasing the release of growth factors (EGF and PDGF) and inhibitory immune factors (soluble anti-PD-L1, GM-CSF, and IL-10). Conclusions: CF33-hNIS-antiPDL1 is oncolytic with immune-activating effects against GCPM ex vivo and has solid potential for intraperitoneal therapy of GCPM patients.