Ampon Tangnoppakul, also known as Ah Kong, died this week in a Bangkok prison hospital where he was being treated for acute stomach pains.
Ampon, 61, was sentenced to 20 years in jail in November after a criminal court found him guilty of lese-majeste -- the controversial Article 112 of the Criminal Code -- and computer crimes, a report in the Bangkok Post said.
Ampon was charged with sending four text messages with offensive content in May last year to the personal secretary of Prime Minister Abhisit Vejjajiva criticizing the queen, the Post reported.
During the trial, Ampon claimed the messages were sent from a phone he didn't have and he didn't know how to send text messages.
Ampon's lawyers said this week their client was hoping to seek a royal pardon.
The government's Corrections Department said Ampon developed acute stomachaches and was admitted May 4 to the Bangkok Remand Prison's hospital where he died May 8.
But Tida Tavornseth, chairman of the political pressure group United Front for Democracy against Dictatorship, said the UDD would seek permission for its lawyers to attend the medical autopsy.
"Ah Kong died too soon," she said. "It's sad because the people want to learn more from this case, which is a matter of international concern."
Tida also said the UDD would bring Ampon's medical treatment and death to the attention of the International Red Cross.
Ampon's death has highlighted Thailand's lese-majeste laws, a focus for the so-called Red Shirts who support the UDD.
Red Shirts have openly clashed in the streets with the government, whose supporters are called Yellow Shirts and who support the People's Alliance for Democracy.
Thailand's lese-majeste laws protect the king of Thailand, as head of state, from criticism, although his immediate political power is limited.
King Bhumibol Adulyadej has reigned as an enthroned monarch since 1946, nearly six years longer than the world's second-longest reigning monarch, Queen Elizabeth II of Britain.
The laws have been heavily criticized as a tool for the government, police and military to clamp down on political dissidents and foreigners.
Last December, Thailand's army -- supporters of the laws -- threw its authority behind local police efforts during the New Year period in case of protests over the lese-majeste laws.
The move is designed to dampen plans by protesters to gather in public and cut off Internet campaigns demanding changes to the laws that prohibit insults to the Thai royal family.
Thai army Cmdr. Gen. Prayuth Chan-ocha said in December people shouldn't be calling on the government and other authorities to amend Article 112.
"Personally, I feel we should not talk about this and I don't want it to go overboard," he said.
"If people think Thai law is unjust or too harsh, they can go live abroad. I would like Thai people to stop confronting each other and creating conflict."
Foreigners also are caught up in Thailand's lese-majeste law.
U.S. citizen Joe Gordon was sentenced in November to five years in jail for translating parts of a banned biography of the king and posting them online. His five-year sentence was reduced because he pleaded guilty during his trial in October.
Gordon, a 55-year-old, Thai-born U.S. citizen, was sentenced in a Bangkok court under the lese-majeste law, even though he posted the offending content online while living in Colorado.
Gordon lived in the United States for about 30 years until May last year when he visited Thailand for arthritis treatment.