The lawyer said his client, Joe W. Gordon, was released Tuesday after receiving the pardon, CNN reported.
"The case is over, he has no more pending court case or obligatory things to do," his lawyer, Anon Rumpa, told CNN.
The case of Gordon, whose Thai name is Lerpong Wichaicommart, was one of several matters human rights advocates cited in their criticism of Thailand's laws that punish defaming, insulting or threatening the royal family, CNN said.
U.S. authorities also expressed concern about Gordon's imprisonment.
Gordon, 54, went to Thailand in 2010 after spending 30 years in the United States. The Asian Human Rights Commission said he was arrested in May 2011 for posting a link on his blog to an unauthorized biography of King Bhumibol Adulyadej. The charge of writing and posting articles insulting the monarchy under Thai law could result in up to 20 years in prison.
CNN said Gordon was taken in by the U.S. Embassy after his release.