The SET index lost 8.31 points today or 0.59 per cent to 1,404.77, on very thin turnover of Bt28.8 billion.
The baht also weakened against US dollar, to 31.60-62, in line with declines in regional currencies.
Prolonged protests are cited as a reason for foreign sell-off in Thai shares and bonds. Another key reason is the anticipation for a remark from the US when the tapering will start.
On Thursday, new Federal Reserve chairwoman Janet Yellen will appear before US senators to defend her nomination as Fed policymakers debate whether the stimulus policy known as quantitative easing is still needed to shore up the world’s largest economy.