The corrosion behaviour of the Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5 bulk metallic glass prepared by copper mould casting with 3mm thickness was studied in borate solutions with pH 5–8.4 and in sodium hydroxide solution with pH 13. A superior corrosion resistance and passivation ability compared to that of Mg and the conventional AZ31 alloy was detected in weakly acidic to weakly alkaline solutions. This is mainly attributed to the beneficial effect of the alloying components, which are homogeneously mixed in the amorphous phase. They cause the establishment of a more noble alloy state and possess a higher tendency for surface passivation than the main component Mg. A high sensibility to chloride-induced corrosion exists, which is at low chloride concentration strongly pH value dependent. For the first time filiform corrosion was detected for Mg-based metallic glasses with filaments propagating randomly on the passivated amorphous surface. Y-rich crystalline defects in the bulk glassy sample seem to have no evident effect on the corrosion processes.