We present [3 + 2] cycloaddition reactions between azides and alkynes on a Au(111) surface at room temperature and under ultrahigh vacuum conditions. High-resolution scanning tunneling microscopy images reveal that these on-surface cycloadditions occur highly regioselectively to form the corresponding 1,4-triazoles. Density functional theory simulations confirm that the reactions can occur at room temperature, where the Au(111) surface does not participate as a catalytic agent in alkyne C-H activation but acts solely as a two-dimensional constraint for the positioning of the two reaction partners. The on-surface azide-alkyne cycloaddition offers great potential toward the development and fabrication of functional organic nanomaterials on surfaces.