Patients with DNA mismatch-repair deficient (dMMR)/microsatellite instability-high (MSI-H) colorectal cancer (CRC) represent a biomarker-defined population with distinct clinicopathological features who are susceptible to immune checkpoint inhibitors (ICIs). However, their survival outcomes vary considerably and nearly half of them exhibit primary resistance to current ICIs, suggesting substantial molecular heterogeneity even among tumors with dMMR/MSI-H. We conducted an extensive analysis of the tumor microenvironment (TME) using multiple transcriptomic, proteomic and immunohistochemistry cohorts of CRC, comprising 222 dMMR/MSI-H and 1440 MMR proficient/microsatellite stable tumors. We developed a TGFβ-responsive stromal gene signature and then identified a unique poor prognostic subgroup of patients with dMMR/MSI-H CRCs, characterized by the upregulation of transcriptional programs, including the TGFβ-rich active TME, angiogenesis, M2 macrophage polarization, and the extracellular matrix signature predictive of ICI resistance. The TGFβ-dependent stromal subset within dMMR/MSI-H tumors exhibiting poor survival outcomes was further recapitulated by proteomic datasets and immunohistochemistry for VCAN protein expressed by cancer-associated fibroblasts. Meanwhile, this dMMR/MSI-H stromal subgroup was enriched neither with CD8+ T cell infiltration nor common genomic alterations, such as mutation density and BRAF mutations, compared with dMMR/MSI-H tumors without TGFβ-dependent stromal activation. In conclusions, this study revealed a novel stromal subgroup of patients with dMMR/MSI-H CRC, demonstrating a TGFβ-rich tumor-promoting TME and unfavorable survival outcomes. Implications: Dual inhibition of immune checkpoints and the TGFβ signaling may offer a promising strategy for those patients.