Ruthenium (Ru) is known as the optimal metal catalyst for ammonia (NH3) synthesis, but the poor light-harvesting capability restricts its application in photocatalysis. Herein, we construct an antenna-reactor nanostructure through the controllable growth of an ultrathin Ru nanocluster shell with desired catalytic activity on the plasmonic gold (Au) nanoantennas. In this nanostructure, Au nanoantennas interact strongly with light to generate hot carriers, meanwhile Ru nanoclusters adsorb and activate N2, leading to the reduction of N2 to NH3 by the generated hot electrons. This antenna-reactor plasmonic photocatalyst exhibits shell-thickness-dependent photocatalytic activity toward nitrogen (N2) photofixation under visible and near-infrared light illumination.