Australian filmmaker James Ricketson was convicted of espionage and sentenced to six years in prison.
The 69-year-old man has already spent 15 months in the Phnom Penh prison since he was arrested for espionage.
After the judge pronounced the guilty verdict, Mr. Ricketson's family said they were devastated by the result.
"The cost of this result, for James, and all our family and friends is immense, it has, and continues to be, a truly brutal experience," said a statement on behalf of the family.
"We will continue fighting, we will not give up."
His trial ended on Wednesday with an additional charge of treason sought by prosecutors.
During the trial, prosecutor Seang Sok said that Ricketson had entered Cambodia to incite hatred, with the aim of overthrowing Hun Sen and his government and providing information to foreign states that was detrimental to the country.
He also said Ricketson, 69, had used his filming as a cover story since 1995 for his espionage activities, citing a dozen emails used to justify the allegations.
"He has to take responsibility before the law with respect to his own illegal activities," he said.
He did not mention any country that Ricketson supposedly spied on, but he sought the additional charge of treason, which Judge Seng Leang said he would consider.
However, Ricketson said that the prosecutor had not presented any witnesses as required by the Cambodian law and that his arrest without a warrant was illegal.
He added that the notion that he was a spy and communicated Cambodian security issues to then-Australian Prime Minister Malcolm Turnbull through a Gmail account was "extravagant and ridiculous."
His defense argued that the evidence did not support the charge and that it was a mistake to imprison a man who had brought "culture and humanity" to the poor of Cambodia.
"James has done a lot of good here, nothing to do with espionage," said lawyer Kong Sam Oun in reference to his charity work. "It's impossible to be a spy for 22 years."