The floods swamped Chawana Uthit Road, Wat Mai subdistrict and Mueang Chanthaburi at 2am on Wednesday.
Water, 70-80cm high, ploughed through roads and past buildings, stranding a number of passing vehicles.
Officials toiled in the dark to evacuate as many people as possible.
Khao Khitchakut Highway section No 3587, from Ban Tham to Yang Rahong, was also inundated, with the water level being 40cm high. Small cars were unable to pass through.
Residents told reporters they had never seen such strong flash floods before. Storm water roared in so fast that they were unable to move their belongings to safety in time.
The Chanthaburi governor set up an evacuation centre at city hall to provide food and drinks to those hit by the sudden flooding.
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