Chachoengsao, commonly called "Paet Riu," has a history dating back to the period of Khmer influence when it was likely under the control of the Lavo Kingdom (the Khmer). Located along the Bang Pakong River, the city was originally known by names reflecting its geographical features. Some historical accounts suggest that the Khmer term "Stung Trang" or "Chatrung Thera," meaning "deep canal," evolved into "Chachoengsao" over time. Other theories propose that the name could have derived from Thai language terms linked to royal military campaigns. Chachoengsao was first established as a town during the Ayutthaya period under King Borommatrailokkanat (r. 1448–1488) as a "fourth-class town," tasked with guarding the capital.

Historically, Chachoengsao played a strategic role in the kingdom’s defense, particularly under King Naresuan (r. 1590–1605), who used it as a gathering point for troops. Its favorable location along the river made it ideal for defensive maneuvers and guerrilla warfare, securing the capital from threats along Thailand's eastern coast. The city’s nickname, "Paet Riu," originates from local folklore and traditions, with some legends attributing it to the large size of local fish, which were traditionally sliced into "eight strips" when prepared. During Thailand's 20th-century governmental reforms, Chachoengsao became a full-fledged province in 1933, and in 1952 it was designated a central regional administrative hub, overseeing eight nearby provinces, underscoring its significance in regional governance.