Chachoengsao's neighboring provinces are (from north clockwise) Prachin Buri, Sa Kaeo, Chanthaburi, Chon Buri, Samut Prakan, Bangkok, Pathum Thani and Nakhon Nayok. It also has a short coast to the Gulf of Thailand.
Chachoengsao or Paet Rio is a province in the Central region. It has an old history back to the reign of King Borommatrailokkanat in the Ayutthaya period. Most of the people have settled by the Bang Pakong River and along canals. “Luangpho Phuttha Sothon” is a centre of faith of the people of Paet Rio. In the past, Chachoengsao was a fourth class city under the Ministry of Defence. During the reign of King Rama I, it was attached to the Ministry of Interior. Until the reign of King Rama V who changed the administration system, Chachoengsao became a city in the Prachin Buri Circle. In 1916, its status was changed from a city to a province. “Chacheongsao” is a Khmer word which means a deep canal. The name “Paet Rio” comes from the story that the city once teemed with giant snake-head fish; up to 8 cuts were required on the sides in making sun-dried fish.