The Hanthawaddy Kingdom (Mon: , [hsawte]; Burmese: ; also Hanthawaddy Pegu or simply Pegu) was the polity that ruled lower Burma (Myanmar) from 1287 to 1539 and from 1550 to 1552. The Mon-speaking kingdom was founded as Ramaadesa (Mon: , Burmese: ) by King Wareru following the collapse of the Pagan Empire in 1287:205206,209 as a nominal vassal state of the Sukhothai Kingdom and of the Mongol Yuan dynasty. The kingdom became formally independent of Sukhothai in 1330 but remained a loose federation of three major regional power centres: the Irrawaddy Delta, Bago, and Mottama. Its kings had little or no authority over the vassals. Mottama was in open rebellion from 1363 to 1388.

Wareru (Mon: ဝါရေဝ်ရောဝ်, Burmese: ဝါရီရူး, Burmese pronunciation: [wàɹíjú]; also known as Wagaru; 20 March 1253 – c. 14 January 1307) was the founder of the Martaban Kingdom, located in present-day Myanmar (Burma). By using both diplomatic and military skills, he successfully carved out a Mon-speaking polity in Lower Burma, during the collapse of the Pagan Empire (Bagan Empire) in the 1280s. Wareru was assassinated in 1307 but his line ruled the kingdom until its fall in the mid-16th century.

Wareru, a commoner, seized the governorship of Martaban (Mottama) in 1285, and after receiving the backing of the Sukhothai Kingdom, he went on to declare independence from Pagan in 1287. In 1295–1296, he and his ally Tarabya, the self-proclaimed king of Pegu (Bago), decisively defeated a major invasion by Pagan. Wareru eliminated Tarabya soon after, and emerged as the sole ruler of three Mon-speaking provinces of Bassein, Pegu and Martaban c. 1296. With his domain now much enlarged, Wareru sought and received recognition by Yuan China in 1298.

Although he may have been of ethnic Mon or Shan background, Wareru's greatest legacy was the establishment of the only Mon-speaking polity left standing after the 1290s. The success of the kingdom helped foster the emergence of the Mon people as a coherent ethnicity in the 14th and 15th centuries. Furthermore, the legal code he commissioned—the Wareru Dhammathat—is one of the oldest extant dhammathats (legal treatises) of Myanmar, and greatly influenced the legal codes of Burma and Siam down to the 19th century.