Ampon Tangnoppakul was convicted by the Bangkok criminal court in November for sending text messages deemed insulting to the monarchy in 2010. The European Union said at the time it was "deeply concerned" by the sentence.
"His wife called me this morning and told me that he has passed away in prison," Ampon's lawyer Anon Numpa told AFP, adding that his client had been hoping for a royal pardon.
The cause of death was still being investigated, according to a doctor at the Corrections Department hospital where Ampon's body was taken from the Bangkok Remand Prison.
"His medical record showed that he used to have mouth cancer and currently his stomach was swollen which is under investigation," he said. "A witness said last night he was fine but this morning he wasn't moving and was already dead."
Ampon, who became known in Thailand as "Uncle SMS", pleaded not guilty during his trial, one in a series under the kingdom's strict lese majeste legislation, which critics say is used to stifle free speech.
"He had come to represent the enormous degree of injustice that was this lese majeste law and yet he wanted nothing more than to be a grandfather and to enjoy his old age," Amnesty researcher Benjamin Zawacki told AFP.