GISTDA reported the flood situation after studying information from the Sentinel-1 satellite.
Of the 3,289,514 rai affected by floods, precisely 526,577 rai are rice fields, the agency said.
It estimated floods have affected at least 758,000 people, damaged some 3,700 houses, and overwhelmed 1,463 kilometres of roads.
The provinces with the largest flooded areas are Nakhon Sawan (650,967 rai), Ayutthaya (430,745 rai), Phichit (402,083 rai), Suphan Buri (356,651 rai) and Sukhothai (269,629 rai), while Nong Bua Lamphu has the least at 287 rai.
GISTDA said it had sent satellite images to various organisations to support their missions in planning and monitoring the flood situation.
The agency has also adjusted its satellite signal reception to monitor the situation in areas that are expected to be flooded.
Those interested in accessing information can visit the http://flood.gistda.or.th website.