A Lady chapel or lady chapel is a traditional British term for a chapel dedicated to "Our Lady", Mary, mother of Jesus, particularly those inside a cathedral or other large church. The chapels are also known as a Mary chapel or a Marian chapel, and they were traditionally the largest side chapel of a cathedral, placed eastward from the high altar and forming a projection from the main building, as in Winchester Cathedral. Most Roman Catholic and many Anglican cathedrals still have such chapels, while mid-sized churches have smaller side-altars dedicated to the Virgin.

The occurrence of lady chapels varies by location and exist in most of the French cathedrals and churches where they form part of the chevet. In Belgium they were not introduced before the 14th century; in some cases they are of the same size as the other chapels of the chevet, but in others (probably rebuilt at a later period) they became much more important features. Some of the best examples can be found in churches of the Renaissance period in Italy and Spain.

It was in lady chapels, towards the close of the Middle Ages, that innovations in church music were allowed, only the strict chant being heard in the choir.