Wat Mahathat (Thai: วัดมหาธาตุ) or the “Monastery of the Great Relic” is located on the City Island in the central area of Ayutthaya at Tambon Tha Wasukri. The temple is situated on the corner of the present Chikun Road and Naresuan Road. The monastery stood on the west bank of Khlong Pratu Khao Pluak, an important canal, which has been filled up somewhere in the early 20th century. In ancient times the temple was likely fully surrounded by canals and moats. The structure has been registered as a national historic site by the Fine Arts Department on 8 March 1935 and is part of the Ayutthaya World Heritage Historical Park.
Wat Mahathat was one of the most important monasteries of the Ayutthaya kingdom, not only because it was the religious centre and enshrined relics of the Buddha, but also because of its proximity to the Grand Palace. It was a royal monastery and the seat of the Supreme Patriarch of the City Dwelling sect till the end of the Ayutthaya period - at par with the Supreme Patriarch of the Forest Dwelling sect, which had its seat at Wat Yai Chai Mongkhon (called Wat Pa Kaew in earlier times).
Wat Mahathat housed before an unusual Buddha image of green stone believed to be made in the Dvaravati style (Mon) dating from 707 - 757 AD. A governor of Ayutthaya got this statue moved to Wat Na Phra Men during the reign of King Rama III, where it still resides in a small viharn next to the ubosot. The main prang of Wat Mahathat survived until the reign of King Rama V. On 25 May, 1904, the main prang collapsed at the level of the niche. The prang fell further apart in 1911 during the reign of King Rama VI. The Fine Arts Department restored it partially. The symmetrical base with staircases on the four sides is all what remains of the once majestic prang.
All temples in the early period of the establishment of Ayutthaya were clearly Khmer styled, consisting primary of laterite structures (instead of sandstone) and bricks, enhanced with stucco. Wat Mahathat consisted basically of a large central prang surrounded by four subsidiary prangs at the four inter-cardinal points, standing on a raised square platform. The quincunx was surrounded by a courtyard and a roofed gallery, lined with a row of Buddha images. Typically for the Ayutthaya period is that often the gallery was penetrated by a monastic structure, being an ordination or an assembly hall, or even sometimes both. An exception to this was Wat Phutthaisawan.
The Viharn Luang or the Royal Assembly Hall of Wat Mahathat stood east of the prang, orientated towards Khlong Pratu Khao Pluak. The rectangular structure was quite large, measuring 40 m by 20 m. The vihara had a front porch (east) which could be reached by tree staircases. There was also an entry into the hall from both sides. Behind the main pedestal were two exits leading down to the gallery. The multi-tiered roof of the viharn was supported by two rows of columns. The hall contained mural paintings of the Vessantara Jataka. Viharn Luang has undergone several restorations in the past as well as in recent times.
During the reign of King Rama VI in the Rattanakosin period, about 1911 A.D., the main prang of the wat collapsed again and looters seized the opportunity to dig for treasure. Only in 1956 A.D. did the Fine Arts Department undertake excavations around the central area of the prang where the relics must have been kept. The relics of the Buddha were found in the stupa within a seven layer reliquary. Other antiquities were recovered as well, including Buddha images, votive tables, covered boxes shaped like fish and golden plaques in the form of animals. All these objects are now at the Chao Sam Phraya National Museum.