Ampon Tangnoppakul was hospitalised last week for a pain in his stomach, the lawyer told the BBC.
His wife found out that he had died when she visited the jail in Bangkok this morning.
The cause of death is being investigated, said a doctor, and it is still not known when Ampon died.
He was convicted in November last year of sending four messages to an official working for then Prime Minister Abhisit Vejjajiva.
He denied the claims and said he did not know how to send a text.
The conviction – the heaviest for a lese majeste case – sparked outrage among rights groups, with Amnesty International describing Ampon as a political prisoner and the European Union expressing ”deep concern”.
Ampon, who became known as ”Uncle SMS”, had been hoping for a royal pardon, his lawyer Anon Numpa said.
Critics say both laws have been increasingly politicised and used to curb free speech in Thailand. Activists have called for the laws to be reformed.
Some Thai academics and writers have fled the country for fear of being denounced.