Abstract The tensile properties of amorphous Mg–23.5Ni (wt.%) ribbons in the temperature range 25–350 °C were studied. The mechanical behaviour of the alloy can be divided into four different intervals depending on the test temperature. In the 25–125 °C interval, the ribbons are brittle or exhibit small ductility. Between 138 and 150 °C, a substantial increase in the yield stress takes place due to the crystallization of a metastable magnesium-rich phase. In the 175–250 °C temperature range the material exhibits maximum elongations. In this interval the microstructure consists of a mixture of Mg, Mg 2Ni and the metastable magnesium-rich phase. Variations in the volume fraction of these phases determine the mechanical properties of the alloy within this interval. Above 250 °C, the equilibrium eutectic microstructure, consisting of Mg and Mg 2Ni phases, is developed prior to the tensile test.