Thailand's annual headline consumer price index (CPI) rose 0.32% in September from a year earlier, less than expected, commerce ministry data showed on Tuesday.
That compared with a Reuters forecast of 0.44% in September, and against August's 0.52%.
September's headline inflation was the lowest level since January and stayed below the Bank of Thailand (BoT)'s 1%-4% target range for a fourth straight month.
The BoT forecast headline inflation of 0.8% for this year while the commerce ministry predicts 0.7%-1.0%.
The annual core inflation rate was 0.44% in September, compared with a forecast of 0.43%, and August's 0.49%.
In January-September, headline inflation was 0.81% and core inflation was 0.54%.
Last week, the BoT left its policy interest rate unchanged at 1.50%, after a surprise cut in August. It will next review monetary policy on Nov 6.