The court in Oryol found Dennis Christensen guilty of extremism, making him the first Jehovah's Witness in Russia to be sent to prison.
Christensen was detained during a police raid on a local prayer meeting he was leading in May 2017.
"I do not agree with this judgment, it's a big mistake," Christensen told reporters after the sentencing in the city of Oryol. His wife Irina Christensen added: "I'm really sad that such a thing is happening in Russia, very sad. The same thing could happen to any of us."
The verdict was met with consternation around the world including from the U.S. Embassy, which expressed its concern and urged Russia to respect individual's religious freedom.
Russia in recent years has used its vaguely worded extremism laws to go after dissenters, opposition activists and most recently religious minorities. Russia officially banned the Jehovah's Witnesses in 2017 and declared the religious group an extremist organization.
Nearly 100 members of the group face charges in Russia, and more than 20 of them are in jail awaiting trial. Before the ban, the world headquarters of the Jehovah's Witnesses claimed about 170,000 adherents in Russia.
Paul Gillies, spokesman for Jehovah's Witnesses, said in an emailed statement that Christensen did not commit any crime and that he was convicted "merely for practicing his Christian faith."
"This verdict reveals just how fragile religious freedom has become in Russia," Gillies said.
The religious group got a glimmer of hope in December when President Vladimir Putin publicly pledged to look into the reported persecution of Jehovah's Witnesses, calling extremism charges against the religion's adherents "nonsense."
But Putin's spokesman Dmitry Peskov, when asked about the case after the verdict, was unable to say if Putin had looked into the matter and had no comment on the ruling Wednesday.
U.S. Embassy spokesperson Andrea Kalan expressed concern.
"Deeply concerned by the six-year sentence imposed on Jehovah's Witness Dennis Christensen," she tweeted. "We agree with President Putin that persecuting peaceful believers is utter nonsense, and call on Russia to respect freedom of religion."
Amnesty International has said that it considers Christensen and other Jehovah's Witnesses on trial as prisoners of conscience.
Christensen's lawyer Anton Bogdanov said that he will likely file an appeal within 10 days after discussing the matter with his client.
"The man was sentenced to six years behind bars, because he, along with others of the same religious beliefs, read the Bible and spread their religious views," Bogdanov said.