Church of St John of the Collachium

The Church of St John of the Collachium was a medieval church built by the Knights Hospitaller in Rhodes, capital of the island of the same name. It was built in the first half of the fourteenth century and dedicated to the order's patron, John the Baptist. It was the conventual church of the Hospitallers, immediately adjacent to the Palace of the Grand Master, and presided over by the order's most senior religious official. It was used for religious services and processions, meetings of the order's chapter general, and for the elections and funerals of its grand masters.

Architecturally, the church was in the Gothic style, 48–50 metres (157–164 ft) long and 15–18 m (49–59 ft) wide. It was expanded in several phases, adding underground areas and a total of eight annexes, including several chapels. At least six Hospitaller grand masters were buried in the church, including Fabrizio del Carretto, whose elaborate funerary slab was placed in its central part. The church became famous for its collection of relics, which included objects associated with John the Baptist, Jesus, the Virgin Mary, and various other saints.

After the Ottoman conquest of Rhodes in 1522, the church became the town's main mosque. The building was damaged by two earthquakes in the nineteenth century, and destroyed on 6 November 1856 by a lightning strike which ignited gunpowder stored in its cellars, killing at least 200 people. In modern times, only small parts of the northern and eastern foundations remain, and the site has been built over with a school. Archaeological investigations of the site took place in 1932 and 1934 under Pietro Lojacono, in 1988, and under Anna-Maria Kasdagli in 1995.