St John's Co-Cathedral is a Catholic co-cathedral in Valletta, Malta, dedicated to Saint John the Baptist. It was built by the Order of St. John between 1573 and 1578, having been commissioned by Grand Master Jean de la Cassière as the Conventual Church of Saint John.

A co-cathedral is a cathedral church which shares the function of being a bishop's seat, or cathedra, with another cathedral, often in another city. Instances of this occurred in England before the Protestant Reformation in the dioceses of 'Bath and Wells', and of 'Coventry and Lichfield'. These two dioceses were each named for both cities that served as bishop's seats.