Armed FSB officers detained Danish citizen Dennis Christensen in the southern Russian city of Oryol in May 2017, shortly after Moscow banned what it said was an extremist organisation.
“We deeply regret the conviction of Dennis Christensen — an innocent man who did not commit any real crime,” said Yaroslav Sivulskiy, a spokesman for the Jehovah's Witnesses in Russia who was in court.
“It is sad that reading the Bible, preaching, and living a moral way of life is again a criminal offence in Russia,” he added.
Christensen, who moved to southern Russia as an adult and is married to a Russian woman, insisted in closing remarks last week that he had “never committed any criminal acts”.
“I hope that today is the day that Russia defends freedom of religion,” he told journalists as he entered the packed courtroom on Wednesday.
An AFP photographer outside the courtroom saw Christensen, 46, being led past a mass of supporters by police officers following the verdict.
The Jehovah's Witnesses, a US-based Christian evangelical movement, will appeal the verdict within 10 days, according to a statement from the organisation's head office.
Rights groups have condemned the trial, with Amnesty International saying it was “emblematic of the grave human rights violations” taking place in Russia.
Authorities in 2017 amended an existing anti-extremism law to mention Jehovah's Witnesses specifically, ordering their dissolution in Russia.
Since then, there has been an intensifying crackdown on believers, with around 90 criminal trials now pending in dozens of regions of Russia, according to the group.
More than 20 properties belonging to the organisation or its members have been confiscated by law enforcement, its US-based office said.
The 33-year-old leader of the Jehovah's Witness organisation in Ivanovo, a city northeast of Moscow, was arrested last week on extremism charges, Russian media reported.
The Jehovah's Witnesses say they number more than 170,000 in Russia, a country of 144 million people where most are Orthodox Christians. Thousands more of their members have fled abroad.
Members of the group — a movement that began in the United States in the 19th century — consider modern churches to have deviated from the Bible's true teachings.
They reject modern evolutionary theory and refuse blood transfusions.
In a report last year, Human Rights Watch accused the Russian authorities of a “sweeping campaign” of harassment and persecution against the movement.
In 2010, the European Court of Human Rights ruled that the dissolution six years earlier of the movement's Moscow branch had violated the right to freedom of religion and association.
The powerful Russian Orthodox Church has spoken out against the group, with one church official branding it a “destructive sect”.