The invention relates to a process for manufacturing a waterproof magneto-rotor (1) made of rare-earth elements, to be used in a synchronous electric motor having a rotor (2) with a permanent magnet and wherein the rotor (2) is solidly fixed to a shaft (3). The process comprises the following phases: a first phase of coating the rotor (2) with a first inner protective layer (12) obtained by a first injection moulding phase; a second phase of coating the first inner protective layer (12) of the rotor (2) with a second outer protective layer (14) obtained by a second injection moulding phase; at least one annular protective barrier (15) being shaped on at least one or both the opposed end surfaces of the rotor (2) during the first injection moulding phase, for preventing water or humidity from seeping between the first inner protective layer (12) and the second outer protective layer (14).