The river breached a temporary flood barrier at the market, just as it had last month. Vendors were again sent scrambling to salvage their wares and personal belongings.
On the Chiang Rai side, business operators and vendors at the Sai Lom Joy Market moved their goods to safety as a precaution, but could open for trade.
The floodwater reached some shops along a road under a bridge crossing the river.
Chiang Rai Governor Narongsak Osotthanakorn instructed Mae Sai district chief Chutidech Maneechan to inspect the sandbag barriers that were erected along the river late last month and make sure there were no areas at risk.
He urged officials to have evacuation plans ready for riverside communities such as Sai Lom Joy Market, Mai Lung Khon, Mueang Daeng and Koh Sai.