Nimid Jaindamani, 45, fled his bed at Vachira Phuket Hospital about 4am, according to prison officials.
Khun Nimid was arrested on fraud charges on April 25 and had been awaiting trial.
Because of TB and a respiratory complaint, he was lodged in the Phuket City hospital on September 28 and had been there ever since.
Escapes by prisoners from hospital on Phuket are not unknown.
Escapes from the jail itself have also been attempted but without success.