Customs-cleared exports unexpectedly dropped in October after two months of gains, according to Commerce Ministry data released on Monday.
Exports fell 4.22% from a year earlier, compared with the median forecast for a rise of 2.15% in a Reuters poll and a 3.4% increase in September. The export value in October was US$17.78 billion.
The export decline was due to slow global trade, weaker shipments of gold and oil products plus last year's high base of comparison, a ministry official said. Imports rose a better-than-expected 6.5% from a year earlier.
Economists had expected a rise of 3.6% after an increase of 5.6% in September. That produced a trade surplus of US$250 million for October, compared with expectations of $1.90 billion and the previous month's $2.55 billion.
Much of the materials that Thailand imports are assembled into completed goods and shipped out again.
Exports to Japan rose 8.9% in October, while shipments to the US fell by 4.7% and an even bigger drop of 9.2% to the European market.
The 10-month export value was $178.25 billion, down 1.02% from the same period of 2015.
Exports, worth about two-thirds of the country's GDP, have contracted for the past three years, frustrating the military government's efforts to revive the economy.
Last week, the state planning agency forecast exports would be flat this year, rather than fall 1.9% as previously projected. It expected the economy to grow 3.2% this year, up from 2.8% last year.