They were also acquitted of another charge of breaching the Computer Crimes Act in a high-profile trial that had sparked widespread condemnation from human rights groups and the United Nations.
Alan Morison and his Thai colleague Chutima Sidasathian, of the Phuketwan news website, had faced up to seven years in jail over a July 2013 article quoting an investigation by the Reuters news agency which said some Thai navy members were involved in trafficking Rohingya Muslims fleeing Myanmar.
"Phuketwan had only presented their (Reuter's) information that had already been published on their website," she added.
The verdict comes after the region's grim people-smuggling trade was dramatically laid bare this year when migrants were abandoned at sea and in jungle death camps by traffickers following a Thai crackdown, a crisis that eventually forced Southeast Asian governments to respond.
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The two journalists had been facing up to two years in jail for criminal defamation and five years for breaching the Computer Crimes Act after the navy sued Phuketwan for defamation in response to their article.
David Crundwell, a chief spokesman for Reuters, said the agency was "pleased" to hear the verdict.
"Reuters wholeheartedly supports the principles of a free press, and the imperative of journalists across the world to publish independent and reliable news," he said.
Speaking to AFP ahead of the verdict Chutima said "the authorities can cover up problems but you can't cover up everything".
Officials have long been accused by human rights groups of turning both a blind eye to, and complicity in, the trade.
A crackdown in May led to the unravelling of vast people-smuggling networks.