Wyn Ellis, a long-term resident of Thailand with British and Thai citizenship, was freed late on Monday after he was detained shortly after arriving from Europe last Thursday.
Ellis is working on a sustainable rice programme for the United Nations in Thailand. He discovered just a few days ago he had been blacklisted, apparently because of a 2009 letter written by the man he had accused of copying his work.
"I am out and I am off the blacklist," Ellis told Reuters on Tuesday after spending four days in a cell with 15 other people.
"It's a relief to be back in my own home. We had a glass of wine last night to celebrate," he said.