BANGKOK—A lawyer for a man sentenced to 20 years for sending text messages deemed insulting to the monarchy says his client has died.

Amphon Tangnoppakul was in his 60s and became known as "Uncle SMS" when a Thai court issued the verdict in his case in late 2011. It was the heaviest sentence ever handed down in Thailand for a so-called lese majeste case.

Amphon's lawyer Anon Numpa said Tuesday that the cause of death was not yet known and he was not sure when his client died. He said Amphon's wife learned about it early Tuesday during a visit to the Bangkok prison where he was being held.