Mucosal-associated invariant T (MAIT) cells are non-classical T cells that are enriched in mucosal tissues and rapidly produce inflammatory cytokines in response to activation by microbial metabolites. MAIT cells have been shown to play a key role in the response to acute respiratory, mycobacterial and HIV infections, but the regulatory mechanisms that control the effector function of MAITs are not well understood. Here, we show that the proinflammatory micro-RNA, miR-155, is important for MAIT cell development and their ability to secrete IFN-γ upon stimulation. In stimulated MAITs, miR-155 expression was significantly up regulated, and lung derived MAITs from mice lacking miR-155 had a diminished IFN-γ response upon stimulation. Lastly, the MAIT cell compartment in miR-155 deficient mice was considerably reduced compared to wild-type counterparts. These results demonstrate the importance of micro-RNAs in MAIT cell biology, and provide direction for future studies involving the regulatory networks controlling MAIT cell development and function.