The Stock Exchange of Thailand index dropped 1.79% on Thursday, while other markets in the region ended weaker as Sino-US trade tensions lingered.
The SET index lost 29.82 points or 1.79% to end the day at 1,634.44, in trade worth 61.64 billion baht.
Thai shares fell for a sixth session in seven and ended at their lowest close since September last year, a day after the central bank held its benchmark interest rate.
PTT Plc fell 3.1% and Airports of Thailand Plc slipped nearly 2%.
The Philippine Index fell 2.3% to 7098.15, slipping deeper into bear territory on persistent foreign selling.
The index is down nearly 22% from a record high touched in January and the last time it fell over 20% was between April 2015 and January 2016, when it shed 25%.
Foreign investors sold Philippine equities worth net US$42.4 million on Thursday. They have net sold about $1.19 billion in shares from Feb 1 to June 20, sparked by a weaker peso that touched a 12-year low earlier this week.
"We breached 7,200 support, so the likelihood is large that the market may retest the 7,000 mark in a month. We can get some rally towards the end of the year due to the traditional window dressing for the quarter-end," said Manny Cruz, an analyst with Asiasec Equities Inc.
Volumes are low and investors are wary with the trade wars happening between the Unites States and China, said Joel Ng, an analyst at KGI securities.
Rising US yields are also triggering fund flows from emerging markets into the United States, he added.
Vietnam shares slipped 1.2% as Petrovietnam Gas Joint Stock Corp and Vincom Retail JSC ended 3.2% and 5.2% lower, respectively.
Singapore shares fell 0.5%, ending near a nine-month low.
Malaysian shares fell 1% to their lowest close since February 2017. The index ended lower for a ninth straight session, its biggest losing streak since early October.