Ampon Tangnoppakul, 61, was sentenced in November to 20 years in prison for sending four critical text messages from his mobile phone in violation of Thailand's lese-majeste laws, The New York Times reported Tuesday.
Articles published in the Thai press discussed the possibility that his mobile phone's identity may have been pirated.
Ampon was repeatedly denied bail despite a history of cancer. The court ruled that his condition "did not appear to be fatal."
His lawyer said in a telephone interview that prison may have worsened his condition. He complained of stomach pains last week and was sent to a prison hospital.
Observers said the death is likely to sharpen debate over the place of the monarchy in society and the laws that create a separate category for crimes against the sovereign.