AbstractSmart pH‐responsive surfaces that could autonomously induce unidirectional wetting of acid and base with reversed directions are fabricated. The smart surfaces, consisting of chemistry‐asymmetric “Janus” silicon cylinder arrays (Si‐CAs), are prepared by precise modification of functional groups on each cylinder unit. Herein, amino and carboxyl groups are chosen as typical pH‐responsive groups, owing to their protonation/deprotonation effect in response to pH of the contacted aqueous solution. One side of the Si‐CAs is modified by poly(2‐(dimethylamino)ethyl methacrylate), while the other side is modified by mixed self‐assembled monolayers of 1‐dodecanethiol and 11‐mercaptoundecanoic acid. On such surfaces, it is observed that acid and base wet in a unidirectional manner toward corresponding directions that are modified by amino or carboxyl groups, which is caused by asynchronous change of wetting property on two sides of the asymmetric structures. The as‐prepared Janus surfaces could regulate the wetting behavior of acid and base and could direct unidirectional wetting of water with reversed directions when the surfaces are treated by strong acid or base. Due to the excellent response capability, the smart surfaces are potential candidates to be applied in sensors, microfluidics, oil/water separation, and smart interfacial design.