FEM1B is recognized for its significant pro-apoptotic function in colorectal cancer; however, its influence and mechanisms regarding apoptosis in immune cells remain inadequately elucidated. In this study, we demonstrated that FEM1B enhances TRAIL-induced apoptosis in Molt-4, Jurkat, THP-1, and U937 cell lines. Notably, the knockdown of FEM1B in transfected cells resulted in a reversal of the observed increase in cell apoptosis. Our findings indicate that FEM1B activates caspase-3 and caspase-8, but not caspase-9, in response to TRAIL stimulation, suggesting its involvement in the extrinsic caspase-dependent apoptotic pathway. Furthermore, we found that FEM1B interacted with TRAF2 and downregulates its expression in Molt-4 and Jurkat cells, thereby diminishing TRAF2's inhibitory effect on caspase-8. In THP-1 and U937 cells, FEM1B was found to upregulate TRAIL-R2, thereby promoting TRAIL-induced apoptosis. Knockout studies in murine models further corroborated that FEM1B facilitates TRAIL-induced apoptosis. These results demonstrate that FEM1B enhances TRAIL-induced apoptosis in T lymphocytes and monocytes through a caspase-dependent mechanism involving TRAF2 or TRAIL receptors.