Chachoengsao (Thai: ฉะเชิงเทรา, pronounced [t͡ɕʰàʔ.t͡ɕʰɤ̄ːŋ.sāw]) is a town (thesaban mueang) in central Thailand, capital of Chachoengsao Province. It is on the banks of the Bang Pakong River. It includes tambon Na Mueang and parts of Ban Mai, Bang Tin Pet, Wang Takhian, and Sothon of Mueang Chachoengsao District. In 2006, it had a population of 60,893.
The town was established in 1549 during the reign of King Maha Chakkrapat of Ayutthaya and originally was a centre for military recruitment. During the reign of King Maha Thammaracha, the kingdom was in a weak condition due to being defeated by the Burmese. Phraya Lawaek, the Khmer king, conscripted Thais from several towns including Chachoengsao to be in his work force.
Chachoengsao is sometimes referred to as "Paet Riu", a name derived from large fish locally caught in the past.