Abstract Purpose: Immunotherapy efficacy data on appendiceal cancer from clinical trials does not exist, due to appendiceal cancer incidence of 0.97 per 100,000. The goal of this study was to preclinically explore the application of immunotherapy in treating appendiceal cancer in a personalized organoid model. Experimental Design: Patient tumor organoids (PTO) were fabricated using unsorted tumor cells with and without enrichment with patient-matched immune components derived from peripheral blood leukocytes, spleen, or lymph nodes [immune-enhanced PTOs (iPTO)]. Organoids were cultured for 7 days, followed by treatment with immunotherapy (pembrolizumab, ipilimumab, nivolumab), and assessed for treatment efficacy. Results: Between September 2019 and May 2021, 26 patients were enrolled in the study. Successful testing was conducted in 19 of 26 (73.1%) patients, with 13 of 19 (68.4%) and 6 of 19 (31.6%) patients having low-grade appendiceal (LGA) and high-grade appendiceal (HGA) primaries, respectively. Immunotherapy response, with increased expression of granzyme B and cleaved caspase 3 and decreased expression of CK20 and ATP activity, was exhibited in 4 of 19 (21.1%) pembrolizumab-treated and 2 of 19 (10.5%) nivolumab-treated iPTOs. Post-immunotherapy cellular viability, in responding HGA organoids to pembrolizumab, decreased to less than 15% (P < 0.05). LGA iPTO treatment responses were observed in pembrolizumab and nivolumab, with an 8%–47.4% (P < 0.05) viability compared with controls. Ipilimumab showed no efficacy in the examined cohort. Conclusions: Immunotherapy shows measurable efficacy in appendiceal cancer organoids. Information derived from immunocompetent organoids may be applied in selecting patients for clinical trial enrollment in rare diseases where preclinical models of disease are lacking.