Suryavarman I (Khmer: សូរ្យវរ្ម័នទី១; posthumously Nirvanapada) was king of the Khmer Empire from 1006 to 1050. Suryavarman usurped King Udayadityavarman I, defeating his armies in approximately 1002. After a protracted war with Udayadityavarman's would-be successor, Jayavarman, Suryavarman I claimed the throne in 1010. Suryavarman was a Mahayana Buddhist who was also tolerant of the growing Theravada Buddhist presence in the Khmer kingdom.
Suryavarman I established diplomatic relations with the Chola dynasty of south India around 1012. Suryavarman I sent a chariot as a present to the Chola Emperor Rajaraja Chola I. It seems that the Khmer king Suryavarman I requested aid from the powerful Chola Emperor Rajendra Chola against Tambralinga kingdom . After learning of Suryavarman's alliance with Rajendra Chola, the Tambralinga kingdom requested aid from the Srivijaya king Sangrama Vijayatungavarman. This eventually led to the Chola Empire coming into conflict with the Srivijiya Empire. The war ended with a victory for the Chola dynasty and Angkor Wat of the Khmer Empire, and major losses for the Sri Vijaya Empire and the Tambralinga kingdom.
His reign lasted some 40 years and he spent much of that time defending it. Known as the "King of the Just Laws," he consolidated his political power by inviting some four thousand local officials to the royal palace and swear an oath of allegiance to him. Suryavarman I favored Buddhism but he allowed the people to continue practising Hinduism. His palace was situated in the vicinity of Angkor Thom, and he was the first of the Khmers rulers to protect his palace with a wall.
Suryavarman I expanded his territory to the west to Lopburi, including the Menam basin in Thailand, and east into the Mekong basin.
Suryavarman probably started construction at Preah Khan Kompong Svay, and expanded Banteay Srei, Wat Ek Phnom, and Phnom Chisor. The major constructions built by this king were the Prasat Preah Vihear, on Dangrek Mountain, and completion of the Phimeanakas and Ta Keo. Suryavarman I also started the second Angkor reservoir, the West Baray, which is 8 km long and 2.1 km wide. It held more than 123 million liters of water. This is the largest Khmer reservoir that survives. There is some indication that Suryavarman I sent a gift to Rajendra Chola I the Emperor of the Chola Empire to possibly facilitate trade.
Suryavarman I died in 1050 and was given the posthumous title Nirvanapada ("the king who has gone to nirvana"), a nod to his Buddhist beliefs. He was succeeded by his sons, Udayadityavarman II, who died around 1066 and Harshavarman III (Sadasivapada). The latter continued the struggle against internal rebellions and fought back assaults from the Chams until his death in 1080.