St John's Co-Cathedral is a Roman Catholic co-cathedral in Valletta, Malta, dedicated to Saint John the Baptist. It was built by the Order of St. John between 1572 and 1577, having been commissioned by Grand Master Jean de la Cassière as the Conventual Church of Saint John. The church was designed by the Maltese architect Girolamo Cassar, who designed several of the more prominent buildings in Valletta.