Alaungsithu or Sithu I (Burmese: အလောင်းစည်သူ [ʔəláʊɰ̃ 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.

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

[7] Upon ascending the throne, Sithu assumed the royal style Sri Tribhuwanaditya Pavarapandita Sudhammaraja Mahadhipati Narapatisithu.

[8] 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)[9] had driven out the rightful heir, who fled to Pagan, where he subsequently died.

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.

Instead of a diplomatic marriage, the chronicle states that Bagan captured a lesser Sinhalese princess, sent by Parakramabahu, on her way to Cambodia sparking a brief naval war.

Having sent Min Shin Saw a small town about 90 miles north of Pagan, Sithu then appointed the second son Narathu as heir apparent.