This disclosure relates to a process for the purification of 2-acrylamido-2-methylpropanesulfonic acid, which comprises dissolving said sulfonic acid in hydrous acetic acid of a water content of 3 to 80% by weight at 60.degree. to 110.degree. C. and subjecting the resulting solution to recrystallization. The purified product thus obtained is useful particularly as a starting material in preparing a high-molecular-weight polymer or copolymer.