Rubble masonry is a facade system using the materials of masonry but constructed without arranging the units in regular courses or patterns. The stones or other units of rubble masonry are generally uncarved and cannot be matched together exactly - therefore the uneven spaces between them are filled with mortar of varying thickness, or left unfilled if the pieces lend themselves to structurally stable stacking. Construction of rubble masonry may require formwork until the mortar dries.

This system is often used in building facades (or on structures like bridges) for an intentionally rustic effect.

Mortar is not mandatory where the pieces can be stacked stably without it. The irregular fit and lack of regular courses distinguish this system from fitted masonry.