Verdict
Ressa believes the charges against her are politically motivated and says she will fight the verdict.
A writer for the site Reynaldo Santos was also found guilty alongside Ressa.
Former CNN journalist Ressa, 56, and her colleague were allowed to remain free on bail, pending a possible appeal.
If the conviction stands, their convictions carry a sentence of up to six years.
"Rappler and I were not the only ones on trial," Ressa told the BBC after the verdict.
"I think what you're seeing is death by a thousand cuts – not just of press freedom but of democracy."
Links
The case concerns an article about a businessman’s alleged links to illegal drugs and human trafficking. The story also said he lent his car to a top judge.
The prosecution came under a "cyber-libel" law which came into force in September 2012 – four months after Rappler published the article.
But prosecutors said a correction to the story in 2014 – to fix a "typo" – meant the article was republished after the law had come into effect.
Judge Rainelda Montea said in court that the allegations had not been proven and added that her verdict was based on evidence presented and that freedom of the press "cannot be used as a shield" against libel.
Freedom of expression
While the 2012 law mainly targets cyber-crime, it was criticised at the time for threatening online freedom of expression and data security.
"Our justice system was on trial today," Ressa said after the verdict.
"And it just joined the kind of messaging that was pushed out on social media in 2016 [when Mr Duterte was elected]: journalist equals criminal."
President Duterte once called Rappler a "fake news outlet" angered by reporting into his punitive policies, and alleged conflicts of interest in his inner circle.
The BBC reports that while freedom of the press is guaranteed under the constitution, the Philippines is one of the most dangerous places in the world for journalists, says US-based Freedom House.
"Private militias, often hired by local politicians, silence journalists with complete impunity," says Reporters Without Borders.
Retaliation
Critics of President Duterte say that since he came to power, the media has been subject to pressure and retaliation from the government, if it criticises the administration too strongly.
"So much is at stake," Ressa told the BBC after the court decision.
"Not just for journalists – but for Filipinos, for democracy.
"I think we're fighting the same battles that journalists all around the world are facing against populist authoritarian leaders that are hitting the messengers."