Thailand fell six places in the latest Corruption Perceptions Index, released on Tuesday by Transparency International.
The country was placed 110th with a score of 35 out of 100 in the 2021 survey, a drop from 104th a year ago, when it also scored one more point.
The index uses a scale of zero to 100, with zero meaning highly corrupt and 100 points meaning almost free of corruption.
The 2021 report did not provide details, but Thailand was far below the global average score of 45.
The anti-corruption agency only said countries in the Asia-Pacific region needed to make more effort to tackle corruption.
Thailand's latest score matched its lowest point in 2013 and 2017. The country's score of 36 was stable for 2019-2020.
Singapore was the top performer in Southeast Asia and fourth least corrupt in the world with a score of 85. Malaysia was also ahead of Thailand.
Denmark, Finland, New Zealand shared the top position with a score of 88, while Norway and Sweden were placed equal with Singapore.
The most corrupt country was South Sudan, at last place among 180 countries. Its score was only 11.
