Wat Phra Si Rattana Mahathat colloquially referred to as Wat Yai, is a Buddhist temple in Phitsanulok Province, Thailand, where it is next to the Nan River, near Naresuan Bridge.
Wat Phra Si Rattana Mahathat, known among the locals as "Wat Yai", was founded in 1357 by King Lithai of Sukhothai. The temple was built at the same time as Wat Phra Phuttha Chinna Si and Wat Phra Si Satsadaram. During the reign of Ekathotsarot of Ayutthaya, its Buddha image was gilded and the temple was later developed by Chulalongkorn and Bhumibol Adulyadej of Rattanakosin. The temple is located at the foot of Naresuan Bridge on the bank of the Nan River. It has an area of 36 rai. The temple is very famous because of its golden Buddha image called Phra Phuttha Chinnarat, which is considered by some Thais to be the most beautiful Buddha image in the country.
The temple's large wihan has an immense main entrance with mother-of-pearl inserts donated by King Boromakot in 1756. Beyond the wihan is a Khmer-style prang, the inside of which can be accessed via a stairway. The prang is said to enshrine relics of the Buddha.