High and medium entropy alloys are a new class of metals with interesting properties. Among them, the CoCrNi medium entropy alloy has attracted considerable attention, due to the excellent combination of mechanical properties, enabled by an early onset of mechanical twinning. However, despite the ongoing interest of the scientific community, the real structural application of these materials is still missing. This is partially caused by the use of quasi-static compression and tensile testing. However, in real-life applications, most of the structural failures are caused by fatigue or creep. In this study, we investigate the fatigue properties of fine-grained CoCrNi alloy doped by interstitial nitrogen. All tests were carried out in load-control mode. The CoCrNiN alloy exhibited fatigue behavior similar to usual FCC alloys with cracks starting from surface extrusions and intrusions. The S-N curve is rather steep. The samples loaded at higher stresses showed the onset of deformation twinning. The alloy has a rather high fatigue limit, probably caused by high critical-resolved shear stress.