Pathein (, pəθèɪ̯ɰ̃ mjo̰; ဖာသီ, phasɛm), formerly called Bassein, is the largest city and the capital of the Ayeyarwady Region, Myanmar (Burma). It is located 190 km (120 mi) west of Yangon within Pathein Township on the bank of the Pathein River (Pathein), which is a western branch of the Irrawaddy River, the city has a population of 237,089 (2017 census). Although once a part of the Mon kingdom, Pathein has few ethnic Mon residents today. The majority are of Bamar with a significant Karen, Burmese Indian, Rakhine and Burmese Chinese populations .
The city's name is believed to derive from the Old Mon name, ([pha sɛm]). "pha" means great or wide and sī/sɛm means river or sea. Pha-sɛm means a big sea. The name was corrupted to Bassein during the British colonial period.
An alternate theory holds that the city's name comes from the classical name of Pathein, Kusimanagara, a name used by ancient writings and the Kalyani inscriptions. Pathein itself is a corruption of Mon "Kuthen," which itself is a contraction of Kusimanagara. This is supported by the fact that the Portuguese call Pathein "Cosmim."
Pathein was part of the Mon kingdom. The British built a fort and established a garrison in 1826, after the First Anglo-Burmese War.
The 1983 census recorded a population of 144,092.
Lying at the western edge of the Ayeyarwady River delta, on the Pathein (Ngawan) River west of Yangon, Pathein is accessible to large vessels and despite its distance from the ocean, it is the most important delta port outside of Yangon. The city is also a terminus of a branch of the main railroad line which connects it to Hinthada, Letpadan and Yangon. In the 1990s, the road network along the Irrawaddy to Monywa was improved.
The coastline along the Bay of Bengal is surrounded by the Arakan Mountains. In the area is Inye Lake, long and wide, which is known for its fishing. There is also an offshore reef at Diamond Island that is popular with bathers. Diamond Island is also noted as a turtle breeding ground.
The city is a rice-milling and export centre.