PETRIOU (also called Cha-chang-sao), a town and port of Siam, in the division of Pachim, about 45 m. E. of Bangkok. It is the centre of that part of southern Siam which is watered by the Bang Pakong River. It is built on low-lying, swampy ground, about 10 m. from the mouth of the above river. The population is about 10,000, mixed Siamese and Chinese, the latter slightly predominating. Rice-mills give employment to a large number of indentured Chinese coolies, but the inhabitants are chiefly engaged in agriculture. A railway connecting with Bangkok was opened in the spring of 1908.