The tip leakage cavitation phenomenon observed in axial-flow type turbines is one of the major causes of blade tip erosion. This study is focused on the Kaplan turbine cavitation occurring at the tip clearance gap between the blade tip and the stationary shroud casing. An unsteady multi-phase cavitation numerical model is studied on a three-dimensional real scale turbine model for five on-cam conditions. The tip leakage cavitation appeared for the optimum cam condition and at high flow rates. The installation of an Anti-Cavity Fin (ACF) on the suction side along the runner blade tip is investigated for its cavitation mitigation functionality. The hydraulic performance, blade loading distribution, vapor volume fraction and tip leakage flow are compared between the blades with and without an ACF. Detailed flow physics on the formation of Tip Leakage Vortex (TLV) and cavitation zones for three on-cam conditions oriented in high flow rates are also presented. The ACF is shown to be effective in decreasing the cavitation intensity at the leading edge without altering the turbine hydraulic performance. A modification of ACF shape at the trailing edge is suggested to avoid damage to the turbine blade and the ACF. •Installing a fin on a Kaplan turbine suction side tip weakens leakage cavitation intensity.•The anti-cavity fin attachment does not affect the turbine hydraulic performance.•The fin proves to be advantageous at the leading edge and unfavorable at the trailing edge.•Trimming of the fin is suggested for large clearance gap at trailing edge.