Khlong Mahachai is a major canal in Samut Sakhon Province. It runs from the town of Tha Kham to the administrative center of Samut Sakhon, by the banks of the Tha Chin River.
Khlong Mahachai was dug by the troops of Ayutthaya under General Chao Phraya Ratchasongkhram in 1705. This took place a year after the barge incident involving the royal barge Ekkachai, which resulted in the steerman Phanthai Norasing being executed on the spot.
Khlong Mahachai was originally called Khlong Sanam Chai. The locals call it Khlong Than. The canal creates a straight shortcut of the curving Khlong Khok Kham canal. At the mouth of Khlong Mahachai is the Phanthai Norasing Shrine.