The Stock Exchange of Thailand index closed lower, while Indonesian shares plunged on Monday as a wider-than-expected August trade deficit and the imminent new round of US tariffs on Chinese goods hurt investor sentiment in the region.
The SET index lost 3.82 points or 0.22% to end the day at 1,718.39, in turnover of 49.52 billion baht. Energy and financial stocks dragged the Thai index marginally lower with oil and gas producer PTT Plc falling 1.5%.
Indonesia's trade deficit narrowed in August, but the gap was larger than expected as exports growth slowed, government data showed on Monday, adding to pressure on the rupiah and local stocks.
The local currency rupiah too was under pressure, and fell 0.5%.
US President Donald Trump is likely to announce new tariffs on about US$200 billion on Chinese imports as early as Monday, a senior administration official told Reuters on Saturday.
Trump's plan comes at a time when his Treasury Secretary Steven Mnuchin's attempts to restart trade talks with another bilateral meeting this month.
Indonesian stock index closed 1.8% lower, its worst trading day in more than a week, with losses across the board.
Telekomunikasi Indonesia was the biggest drag on the benchmark, falling 3.3%, while conglomerate Astra International lost 3.8%.
The index of the country's 45 most liquid stocks fell 2.4%.
Singapore shares slid 0.6% after three sessions of gains, with United Overseas Bank falling 1.3% and Jardine Matheson Holdings declining 0.5%.
Singapore's on-year non-oil domestic exports (NODX) growth slowed sharply in August as sales of pharmaceuticals came off the sizzling pace of the month before, official data showed on Monday.
Philippine shares were largely unchanged as gains in real estate and consumer staples were offset by losses in industrial stocks.
Malaysian financial markets were shut for a holiday.