A sanctuary lamp is a red light which is hung or displayed in the Churches of those Christian faiths who believe in Transubstantiation. It is placed, often in a gold or brass candelabrum in or near the vicinity of the altar, indicating that anyone approaching the altar should genuflect (= bow by bending the knee and inclining the head), to show respect for Christ, who according to the theology behind substantiation, is present in the communion wafers in the Tabernacle. The belief that Christ is physically and not just symbolically present in the tabernacle is called the Real Presence.

A sanctuary lamp in a Roman Catholic Church