Alaungsithu or Sithu I (Burmese: အလောင်းစည်သူ [ʔəláʊɰ̃ sìðù]; also Cansu I; 1090–1167) was king of the Pagan Dynasty of Burma (Myanmar) from 1112/13 to 1167. Sithu's reign was a prosperous one in which Pagan was an integral part of inland and maritime trading networks. Sithu engaged in a massive building campaign throughout the kingdom, which included colonies, forts and outposts at strategic locations to strengthen the frontiers, ordination halls and pagodas for the support of religion, as well as reservoirs, dams and other land improvements to assist the farmers. He also introduced standardized weights and measures throughout the country to assist administration as well as trade. He presided over the beginning of a transition away from the Mon culture toward the expression of a distinctive Burman style.

Sithu is remembered as a peripatetic king who traveled extensively throughout his realm, built monuments and nurtured Theravada Buddhism with acts of piety.

Sithu was born Zeyathura Sithu (Burmese: ဇေယျသူရ စည်သူ, Pali: Jayyasura Cansu) to Saw Yun (son of King Saw Lu) and Shwe Einthi (daughter of King Kyansittha) on 17 January 1090. (According to Zatadawbon Yazawin, he was born on 13 December 1089.) The chronicles do not agree on the dates regarding his life and reign. The table below lists the dates given by the four main chronicles.

At Sithu's birth, Kyansittha, who thought that he had no son, was so delighted that he crowned the infant as king, and presented the baby to the people saying "Behold your king! Henceforth, I reign only as his regent." (It turned out that Kyansittha did have a son by a wife during one of his exiles in the 1070s. That son, Yazakumar, made no claim to the throne.)

Sithu faced no opposition to the throne after his grandfather, Kyansittha, died in 1112. He was the great-grandson of Anawrahta on his father's side. His coronation was presided by an aging Primate Shin Arahan who also presided the coronations of the two predecessor kings, and adviser to three previous kings. Upon ascending the throne, Sithu assumed the royal style Sri Tribhuwanaditya Pavarapandita Sudhammaraja Mahadhipati Narapatisithu.

The early part of Sithu's reign was spent repressing revolts, especially in Tenasserim and north Arakan. A Pali inscription found at Mergui (Myeik) is evidence that Tenasserim then paid allegiance to the Pagan monarchy. In north Arakan, a usurper (Kahton, lord of Thets) had driven out the rightful heir, who fled to Pagan, where he subsequently died. Pagan's initial attempt to restore the rightful heir Letya Min Nan—a combined land and seaborne invasion—failed but the second attempt in 1118 succeeded. (The Arakanese chronicles report the date as 1103.) Letya Min Nan, in gratitude, repaired the Buddhagaya shrine in the honor of his overlord Sithu.

Sithu traveled far and wide throughout his dominions, building many works of merit. These pious pilgrimages form the main theme of the chronicles of his reign. He reportedly sailed as far south as Malaya and Bengal in the west. Like his great-grandfather Anawrahta, he also traveled to Nanzhao Kingdom. There was apparently much disorder during his long absences from the capital.

The rulings given at his court, some of which by himself, once existed in a collection, the Alaungsithu Hpyatton.