Prime Minister Prayut Chan-o-cha confirmed on Wednesday that his chief adviser Thosaporn Sirisumphand had resigned from the post.
"Yes, he quit. Quitting is quitting," Gen Prayut said when asked about the resignation of Mr Thosaporn, who was his chief adviser.
The prime minister did not go into detail on when the resignation took effect.
Gen Prayut has been tightlipped for days, with his spokesmen saying the prime minister wanted to concentrate on the flooding and other problems.
Mr Thosaporn on Oct 12 submitted his resignation as chairman of PTT Plc effective from Saturday. He did not give any reason for the decision.
He retired as secretary-general of the National Economic and Social Development Council in 2020 and was appointed the prime minister's chief adviser the same year.
Mr Thosaporn is known to be one of Gen Prayut's the most trusted economic advisers.
Deputy Prime Minister Prawit Wongsuwon was apparently not aware of the resignation of Mr Thosaporn, and seemed surprised when the name was mentioned.
"Who resigned?" Gen Prawit said, when asked about Mr Thosaporn's decision. "Who's he?" he added, before getting into his limousine.
Mr Thosaporn's resignation comes amid speculation that Gen Prayut will soon make changes to his cabinet lineup.
The prime minister said on Oct 6 that the Democrat Party wanted to fill its deputy interior minister portfolio left vacant after Niphon Bunyamanee stepped down recently to defend a court case.