Nonadiabatic effects on the reaction rate constant of a model phenol-amine proton transfer system in a nanoconfined solvent have been investigated by employing classical mapping in conjunction with a reactive flux approach. It is observed that allowing nonadiabatic transitions makes the transition state more accessible thermodynamically but decreases the reactive flux due to increased transition state recrossing, resulting in an overall reduction in the rate constant by more than a factor of 2. The physical origins of these features are discussed.