Suriyan Sucharitpolwong, also known as Mor Yong, died of septicaemia late on Saturday, the justice ministry said.
He was among three men arrested last month on lese majeste charges, and the second in the group to have died.
The deaths have raised questions over the military government's high-profile crackdown on lese majeste cases.
Authorities said Mr Sucharitpolwong, 53, appeared to have health difficulties in the days leading up to his death. He was found unconscious in his Bangkok cell and taken to hospital on Saturday, where he later died, a justice ministry statement said on Monday.
A post mortem examination later found he had a blood infection, said Justice Minister Paiboon Koomchaya.
The fortune teller was last seen in public in court on 21 October along with his assistant and a police officer, where all three were accused of royal defamation and of exploiting ties with the royal family.
The police officer, Prakrom Warunprapa, was found dead two days later while in military custody. Authorities said he hanged himself.
Since its coup last year, the military government has arrested a string of suspects, accusing them of claiming or using connections to the monarchy for personal benefit.
Critics have said the broadly-worded lese majeste laws are being used to silence dissent and opponents.
A celebrity fortune-teller detained in Thailand for allegedly insulting the royal family has died in military custody, the authorities say.