The man denies the charge against him, according to the Thai-language prachatai.com news website, which tracks lese majeste cases. The website said he was born in Thailand, lived in the U.S. state of Colorado for 30 years and returned to Thailand last year for medical treatment.
Thailand is a constitutional monarchy but has severe lese majeste laws that mandate a jail term of three to 15 years for any person who "defames, insults or threatens the king, the queen, the heir to the throne or the Regent."
Thai-based media routinely self-censor coverage of the royal family, but the Internet has tested the taboo. Thai authorities have responded by trying to block more than 2,000 websites.
Critics say the lese majeste laws are often a weapon of political harassment, and calls have grown recently to amend or abolish them.
The DSI spokeswoman, who spoke on customary condition of anonymity, said the American is jailed in Bangkok after a Thai court denied his request for bail. She said he was also accused inciting public unrest under Thailand's Computer Crimes Act. DSI did not identify him.
Police arrested the man at his home in the northeastern province of Nakhon Ratchasima, and seized a computer and cell phone, praachatai.com said.
A U.S. consular officer visited the American on Friday morning, said Walter M. Braunohler, the U.S. Embassy spokesman in Bangkok.
Braunohler gave no other details and did not identify the man, saying only that "we're still looking into what the exact charges are."