The Tha Chin river is a distributary of the Chao Phraya river, Thailand. It splits near the province of Chai Nat and then flows west from the Chao Phraya through the central plains, until it empties into the Gulf of Thailand in Samut Sakhon Province. The Tha Chin river has many regional names. While passing Nakhon Pathom it becomes the Nakhon Chai Si river. The name Tha Chin is the convention used in most scientific documents. Tributaries of the Tha Chin include Huai Krasiao, Huai Mae Thawip, Khlong Chorakhe Sam Phan, and Khlong Bang Len. The Tha Chin drains a total area of 13,681 square kilometers (5,282 sq mi). The Tha Chin Basin is part of the Chao Phraya Watershed.
The Thai Pollution Control Department (PCD) has reported that the water quality of rivers flowing into the upper Gulf of Thailand has seriously deteriorated in the past decade. The department found the river contains bacteria and nutrient pollution from phosphates, phosphorus, and nitrogen causing algae to grow faster than ecosystems can handle, harming water quality, food resources for aquatic animals, and marine habitats. It also decreases the oxygen that fish need to survive.