The hydroboration of alkynes with bis(pinacolato)diboron is a useful method for the synthesis of vinyl boronate esters, which are essential intermediates in organic syntheses. Copper catalysts have been used extensively in these reactions. However, previously reported Cu-catalyst systems inevitably require additives and elevated temperatures. Herein, we report, for the first time, a simple and efficient hydroboration of alkynes under additive-free and mild reaction conditions (i.e., at a temperature of 30 °C) using a copper nitride nanocube (Cu3N NC) catalyst. A wide range of alkynes can be transformed into their corresponding boronate esters. Cu3N NCs are also applicable in the hydroboration of alkynes with tetrahydroxydiboron to synthesize vinyl boronic acids. Moreover, the Cu3N NCs were easily separated by simple filtration and could be reused several times without any loss of their original activity. Hence, these highly active and reusable Cu3N NC catalysts offer an environmentally friendly route for the efficient production of vinyl boronates.