We report the observation of an unexpected magnetic transition in the Kondo semimetal CeNiSn under uniaxial pressures. For P∥a, both the specific heat C and magnetic susceptibility χ decrease steadily with increasing P. By contrast, once P∥b and P∥c exceed 0.13 GPa, C(T) exhibits a distinct jump at 4 K followed by a peak at 3 K. The concomitant peaking in χ(T) for P∥B∥c is indicative of an antiferromagnetic transition.