Foreign investors sold Thai shares ahead of a three-day weekend and weakness in the oil market spurred selling in energy, with shares in PTT group leading the way.
The benchmark SET index closed down 1.02 percent at 1,227.41, dragged down by energy firm PTT Pcl and PTT Global Chemical Pcl.
After the market's steady rally that pushed the main SET index to a new 16-year high, market players now appeared a little cautious, traders said.
Technical signs pointed to a market correction to 1,150 and 1,100 in the next few months before it could move up again, said Andrew Yates, head of international equity sales at broker Asia Plus Securities.
Thai stock market is shut on Monday for a national holiday, reopening on Tuesday.
1643 (0943 GMT)
(Reporting by Viparat Jantraprap in Bangkok; email@example.com)