Reactive High Power Impulse Magnetron Sputtering operated in multi-pulse mode (m-HiPIMS) of a pure Ti target in Ar/N$_2$/O$_2$ gas mixture (mass flow rates of 50, 2 and 0.16 sccm, respectively) has been used for the deposi-tion of titanium oxynitride (TiO$_x$N$_y$) thin films with variable content of nitrogen (from 0.6 at.% to 24.2 at.%). Increase of the nitrogen content in the deposited TiO$_x$N$_y$ thin films determined a decrease of the optical bandgap energy and a corresponding increase of visible light adsorption. The photocatalytic activity for water molecule splitting of the films deposited on metallic substrate, which were used as the photo-anode in an electrochemical cell, has been investigated by measurements of photoelectrochemical current intensity versus biasing voltage during on/off cycles of visible light irradiation (sun light simulated by a xenon lamp). The as-deposited films have a short range order corresponding to rutile and anatase structures and showed very weak photocatalytic activity and chemical instability in the electrolyte of the photoelectrochemical cell. However, a post-deposition an-nealing treatment of the film with low content of nitrogen (0.6 at.%) improved considerably the visible-light photocatalytic activity, the film crystalline order and chemical stability.