"It's not right for one country to use its yardstick to judge the performance of another country," foreign affairs permanent secretary Sihasak Phuangketkeow told a press conference at the Information and Communication Ministry in Bangkok on Saturday.
"The decision is most regrettable .... Thailand is disappointed and respectfully disagreed with the State Department's decision," he said.
"I do know Thailand is doing better [in fighting human trafficking than the other countries in the Tier 3 category]," he said.
Mr Sihasak also urged the State Department to reconsider its downgrade for Thailand, given the efforts Thailand have made in the past years.
Thailand believes it had made tangible progress in combatting human trafficking in Thailand, he said.
"Ridding the country of human trafficking is a national priority," he said, adding Thailand stood ready to do more to prevent and suppress human trafficking.
He noted that the report was conducted from April last year to this April so it had nothing to do with recent political developments in Thailand.
In any case, the National Council for Peace and Order has reaffirmed its support for all involved agencies to prevent and suppress human trafficking, he added.
Mr Sihasak concluded by saying human trafficking "is a scourge we all face, it's a challenge to mankind".'Disappointing'
The US downgraded Thailand, Malaysia and Venezuela on Friday to its list of the world's worst centres of human trafficking, opening up the countries to possible sanctions and dumping them in the same category as North Korea and Syria.
The three countries were all downgraded to the lowest Tier 3 status in the US State Department's 2014 Trafficking in Persons Report as they did not fully comply with the minimum standards for the elimination of trafficking.
It came as a shock to Thailand, which had repeatedly expressed confidence their country would be upgraded, submitting a 78-page report to the US government in April to make its case.
The downgrades could cause some multinational companies to reconsider investments in industries accused of using trafficked labour such as fisheries, a lucrative business in Thailand, the world’s largest exporter of shrimp.
The countries could also lose US non-humanitarian and non-trade-related aid, and they could face US opposition to help from international institutions such as the International Monetary Fund and World Bank.