Immune checkpoint inhibitors (ICIs) have transformed outcomes in microsatellite instability-high (MSI-H) deficient mismatch repair (dMMR) cancers. The influence of germline MMR gene variants on ICI remains unclear. In this single-center study, 93 patients with Lynch syndrome-associated MSI-H/dMMR digestive cancers received ICI monotherapy or combination therapy. Germline MMR variants were classified, and progression-free survival (PFS) was evaluated by gene and variant type. Most patients carried MLH1 or MSH2 variants. At 24 months, estimated PFS was 81.5% for MLH1, 67.2% for MSH2/EPCAM, and 78.6% for MSH6, with no PFS events in MLH1 promoter methylation or PMS2 subgroups. No statistically significant differences in PFS were observed between gene groups. Performance status ≥ 2 was the only factor associated with poorer PFS. Neither MMR gene type nor variant class significantly influenced ICI efficacy in Lynch syndrome-related MSI-H/dMMR digestive cancers, supporting the use of ICIs regardless of germline MMR genotype.