Maintaining postharvest quality of summer squash (Cucurbita pepo L.) during refrigerated storage remains challenging due to physiological disorders, texture loss, and disease development, which collectively limit marketable life. This study evaluated the effectiveness of passive modified atmosphere packaging (MAP) in preserving the quality of two commercially harvested summer squash fruit types, cocozelle and zucchini, differing in fruit morphology, across three independent storage trials. Passive MAP atmospheres stabilized at approximately 8–15% O₂ and 7–13% CO₂ during storage at 7.5–8.0 °C for up to 21 days. Genotype-dependent differences in postharvest performance and MAP response were clearly observed. Cocozelle fruit developed pronounced chilling injury (CI) symptoms under air storage, whereas zucchini exhibited inherently low susceptibility to CI. In cocozelle, passive MAP consistently reduced CI severity and significantly decreased disease incidence and severity compared with air-stored controls. Although MAP resulted in limited reductions in external firmness in cocozelle, pulp firmness and overall fruit quality were maintained. In zucchini, CI symptoms remained minimal across treatments; however, passive MAP further reduced disease development and preserved marketable quality. The consistent performance of passive MAP across genotypes and independent trials demonstrates its robustness under variable conditions. These results indicate that, under simulated commercial conditions, passive MAP atmospheres within the tested gas ranges effectively mitigate quality deterioration during refrigerated storage, supporting their practical application for extending the postharvest life of summer squash.