At the close, the index was at 1,516.24 points, rebounding from a big loss in the morning session. The index gained 26.03 points or 1.75 per cent from yesterday's closing.
At the open, the index slumped to 1,483.75 points.
Thai baht also weakened to 30.62-65 per US dollar.
Investors are anxiously awaiting the US Federal Reserve's decision on monetary easing. There has been speculation that the Fed may scale down bond purchases, which would lead to lower liquidity and lower appetite for risk assets.
Foreign investors were net-sellers today, with net-sells worth Bt3.6 billion. Their month-to-date net-sells totalled Bt20.9 billion, pushing the year-to-date net-sells to Bt41.98 billion.