The invention relates to a novel process for preparing N-methylnaltrexone bromide, comprising at least the steps consisting in: (i) reacting N-methylnaltrexone methyl sulfate in aqueous solution with an alkaline agent chosen from the group constituted by sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, cesium carbonate, strontium carbonate and mixtures thereof, for a pH of the aqueous reaction medium of between 7 and 10, and then in (ii) reacting the product thus obtained with hydrobromic acid, which is added for a pH of the aqueous reaction medium of between 0.5 and 5, in order thus to obtain the N-methylnaltrexone bromide.