Provided are a proton-conductive composite electrolyte, a membrane-electrode assembly, and a fuel cell in which an improvement of the proton conductivity, and suppression of crossover and insolubilization are satisfied at the same time. The proton-conductive composite electrolyte includes an electrolyte having a proton-dissociative group (—SO3H) and a compound having a Lewis acid group MXn−1, wherein the Lewis acid group and the proton-dissociative group are interacted with each other. The compound having the Lewis acid group is a Lewis acid compound MXn or a polymer having a Lewis acid group MXn−1. The electrolyte having a proton-dissociative group is a fluorine-containing electrolyte, an electrolyte composed of a hydrocarbon-based resin, an inorganic resin, a hybrid resin of an organic resin and an inorganic resin, or the like, or a fullerene compound. A membrane-electrode assembly in which catalyst electrodes are closely in contact with both surfaces of the proton-conductive composite electrolyte is preferably used in a fuel cell.