
September vehicle sales stood at 47,881 units, down 10.5 per cent year on year.
Commercial car sales were hardest hit, falling 22.8 per cent to 29,194 units, dragging down overall figures despite passenger-car sales rising 19.1 per cent to 18,687 units.
Overall vehicle sales in the first nine months of the year slipped 2.2 per cent, or 461,258 units.
"The market shrank due to the worsening domestic political situation coinciding with the global financial crisis affecting Thai consumer sentiment," Toyota Motor Thailand said in its monthly release on behalf of the industry.
"But brisk sales of passenger vehicles offset the plunge in other sectors."
Protesters have besieged government offices since late August, demanding the government step down over corruption claims. On October 7, they clashed with police outside of Parliament, leaving two dead and hundreds injured.
The car industry's statement said it expected better figures for this month, when new car models were introduced.
Three Japanese car giants - Toyota, Isuzu and Honda - continued to lead the market last month, with combined sales accounting for 79.5 per cent of all sales.
Toyota has lowered its sales target for the year from 700,000 units to 650,000, which would be up 3 per cent from last year.