The efficacy of immunotherapy in colorectal cancer (CRC) hinges upon a comprehensive understanding of how the immune system interacts with tumor cells within the colorectal microenvironment. Mature tertiary lymphoid structures (mTLSs) are associated with an increased objective response rate, progression-free survival, and overall survival in patients with CRC. Thus, it has been suggested that increasing mTLSs in the context of CRC could improve patient outcomes. However, no established method to specifically induce TLS maturation within and around tumor sites is available. To address this gap in technology, we engineered a strain, SLC, to express tumor necrosis factor (TNF) superfamily member 14 (TNFSF14, also called LIGHT). This strain colonized tumors and released LIGHT, which then formed a ligand-receptor pair with herpes virus entry mediator (HVEM) to induce a powerful cellular immune response. Furthermore, this engineered microbe modulated the proportions of intestinal innate lymphoid cells (ILCs), which serve an anti-infection role in innate immunity. Mice that were deficient in HVEM or ILC3 exhibited fewer mTLSs, a greater bacterial burden, and increased mortality in two different models of CRC. Thus, this engineered microbe with enhanced immunogenic properties demonstrated the potential to stimulate mTLS-associated antitumor immune responses in the colon and was well tolerated in vivo. Our results indicate that LIGHT-HVEM signaling on group 3 ILCs (ILC3s) is crucial for mTLS formation and T cell-mediated antitumor immunity in CRC and additionally suggest a synbiotic-based therapeutic approach for the management of CRC.