An adapter for mounting electrical outlet faceplates designed for mounting on rectangular junction boxes in hollow walls onto solid walls with cylindrical cavities and sleeves. Junction box mounting is popular in North America, whereas cavity mounting is popular in Europe and the Middle East. Such an adapter provides universal mounting for specialized faceplates which are designed and intended for junction box mounting. The adapter features clamps for gripping the inner surface of a wall cavity or sleeve and a rectangular plate for mounting an faceplate designed for junction box mounting. The rectangular plate has mounting points with a nominal center-to-center distance of 31/4 inches, corresponding to the requirements of junction box mounting.