The headline consumer price index (CPI) dropped 1.57% in June from a year earlier, a less than expected decline, commerce ministry data showed on Friday.
The reading compared with a forecast for a fall of 2.80% in a Reuters poll, and May's 3.44% decline.
The core CPI index dipped 0.05% from a year earlier, the first drop in more than 10 years. That compared with a forecast of 0.0%, and May's 0.01% rise.
The core CPI index, which strips out food and energy prices, dipped 0.05% from a year earlier, the first drop in more than a decade. That compared with a forecast of 0.0%, and May's 0.01% rise.
Headline inflation is likely to be negative every month in the second half due in part to last year's high comparative figures, ministry official Pimchanok Vonkorpon told a briefing.
"The main factor pressuring inflation in the second half is the tourist sector because there will not be many foreign tourists," she said.
A Thai tourism body on Tuesday said the country would see at most 8 million foreign tourists this year, down 80% from a year earlier, as the coronavirus pandemic hits global travel.
Last year's 39.8 million foreign tourists spent the equivalent of 11% of Thailand's GDP. The central bank expects just 1 million tourists in the second half.
In the January-June period, the headline CPI fell 1.13% year-on-year while the core index rose 0.32%, the commerce ministry said.
It forecast the headline CPI to fall between 0.7% to 1.5% this year, Ms Pimchanok said.