The Senate on Monday voted to reject the nomination of Udom Mangmeedee, a former Supreme Court judge, as a member of the Public Sector Anti-Corruption Commission (PACC).
The vote against Mr Udom was 67-51.
Anurak Niyomvech, an appointed senator and member of a committee that examined the nominee's record, said this was the first time the Senate had rejected the nomination of a person to sit on an independent organisation under the constitution.
The rejection of Mr Udom's nomination would be reported to the cabinet, which would be required to start the selection process all over again, he said.
Sources said the senators who voted against Mr Udom, who is 72, were also opposed to the government because the former judge had appeared on the stage at a United Front for Democracy against Dictatorship (UDD) rally and had shown a negative attitude to the Thai judicial process.