Alaungsithu or Sithu I (Burmese: အလောင်းစည်သူ [ʔəláʊɴ sìðù]; also Cansu I; 1090–1167) was king of 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 in-land 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 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: ဇေယျ သူရ စည်သူ, Pali: Jayyasura Cansu) to Saw Yun (son of King Sawlu) 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 claims of the throne.)

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