The Lewis acid catalyzed bromination of aromatic side chains was achieved efficiently by using 1,3-dibromo-5,5-dimethylhydantoin (DBDMH) as the bromination reagent under mild conditions. Zirconium(IV) chloride showed the highest catalytic activity for this benzylic bromination. It was revealed that the present Lewis acid catalysis proceeds by the radical-generation pathway. In contrast, Brønsted acids promoted aromatic-ring bromination without any benzylic bromination. Monobromination of tetramethylsilane was also demonstrated with zirconium(IV) chloride and DBDMH to provide the desired product in good yield.