According to Bank of Thailand's data, at 8.26am, the dollar/baht level was 0.67 per cent weaker from the end-July level.
Baht also weakened against the euro and Japanese yen; down by 0.59 pe rcent from the previous closing to 32.27 per 100 yen and down by 0.52 per cent to 42 against euro.
Treasurers attributed the weakening to poorer-than-expected economic figures in the first half.