Photocatalytic N2 fixation has attracted substantial attention in recent years, as it represents a green and sustainable development route toward efficiently converting N2 to NH3 for industrial applications. How to rationally design catalysts in this regard remains a challenge. Here we propose a strategy that uses plasmonic hot electrons in the highly doped TiO2 to activate the inert N2 molecules. The synthesized semiconductor catalyst Mo-doped TiO2 shows a NH3 production efficiency as high as 134 μmol g-1 h-1 under ambient conditions, which is comparable to that achieved by the conventional plasmonic gold metal. By means of ultrafast spectroscopy we reveal that the plasmonic hot electrons in the system are responsible for the activation of N2 molecules, enabling improvement of catalytic activity of TiO2. This work opens a new avenue toward semiconductor plasmon-based photocatalytic N2 fixation.