St. Johns Co-Cathedral () is a Roman Catholic cathedral in Valletta, Malta. It was built by the Knights of Malta between 1573 and 1578. It was commissioned (ordered to be built) in 1572 by Grand Master Jean de la Cassière. He made it the main church of the Order of the Knights Hospitaller of St John, also known as the Knights of Malta. The Church was designed by the Maltese architect Glormu Cassar (Girolamo Cassar). The church is considered to be one of the finest examples of high Baroque architecture in Europe.