The hydantoin moiety is an important pharmacore, and when halogenated, hydantoin derivatives act as excellent biocides. However, there have been no computational studies concerning the chlorination mechanism for the hydantoin moiety reported. Herein we describe a computational mechanistic study of the chlorination of 5,5-dimethylhydantoin (H) at the B3LYP/6-311+G(2d,p) level. Under a 1:1 molar ratio of hydantoin and a chlorinating agent (HOCl), conproportionation is calculated to be favorable to give the N1 monochloro derivative as the major predicted product, which is in agreement with experiment. Initial direct chlorination at the N1 position is prevented by a high kinetic barrier. The first step involves the deprotonation of the hydantoin moiety (at the N3 position) which is followed by a S(N)2 step transferring a chloronium ion (Cl+) from HOCl to the ionized hydantoin anion. A mechanism is proposed where the N3 nitrogen is chlorinated first followed by the N1 position to form the dichloro derivative. When CPCM solvation free energies (AG(solv)) were added to the gas-phase free energies (AG(gas)) along the S(N)2 reaction path, a sudden decrease in free energy was observed due to the incipient formation of the hydroxide ion. Explicit consideration of solvation within a box of 512 water molecules led to a much more gradual free energy change along the reaction path but a very similar free energy of activation.