The In Buri police station was alerted at 5.25pm that a local fisherman had rescued a seven-year-old boy from drowning in the river.
The boy was seen struggling in the water and the fisherman rushed to pull him to safety.
He said he was swimming with two friends of whom there was no trace.
Police and rescue officials searched but have yet to find the two youngsters, who have been identified as Chinathip Sri-amphai, 9, and Dechit Pumpuang, 7.