Light-assisted N2 fixation to NH3 under mild conditions is attracting massive attention to circumvent energy crisis and global warming. Herein, highly dispersed Cs-decorated Ru sub-nanometric clusters supported on ZrO2 NPs have been prepared by means of MOF-templated transformation method. The obtained catalyst exhibited N2 hydrogenation activity of 1.6 mmolNH3 · gcat-1 · h-1 in the dark (350 oC, 0.1 MPa). Remarkably this value increased by over 300 %, reaching 5.1 mmolNH3·gcat-1·h-1 (204 mmolNH3 · gRu-1 · h-1) under 1 Sun power illumination (1080 W/m2), with an estimated quantum efficiency of 39 %. The influence of the incident light wavelength has been evaluated, revealing 81 % and 213 % enhancement in the NIR and visible region, respectively. DFT calculations were carried out to understand the static adsorption states of Ru sub-nanoclusters. Mechanistic studies have confirmed the co-existence of a photothermal and a nonthermal-hot electron mechanism. XPS, PXRD and FTIR analysis have determined that the Cs species block surface Zr4+ acidity, increasing the basicity of the ZrO2 support. Moreover, partially reduced Cs+ (0<<1) species surrounding the Ru active sites are donating electron density to the adjacent Ru sites, favoring N2 adsorption. Finally, this photocatalyst has shown an extended stability for 100 h irradiation under continuous flow.