The Bank of Thailand has found fake Bt1,000 banknotes in the system.
In a statement, it spotted two fake banknotes, which showed no watermarks on HM the King's photo.
It said police has been alerted and it would start a scheme to raise awareness for fake banknotes.
It said authentic banknotes are printed with special paper which is thicker. The watermarks are visible in bright light. Thanks to the special ink, the figures on the notes can change colours at different angles.