Alleged arms dealer Viktor Bout is on his way to the United States from Thailand after being extradited.
He was flown out of Bangkok shortly after the Thai cabinet backed the extradition request after months of legal wrangling.
Mr Bout was arrested by US agents posing as Colombian Farc rebels after he allegedly tried to sell them weapons in a Bangkok hotel in 2008.
The Russian national now faces trial for conspiring to sell weapons.
If convicted, he faces a maximum penalty of life in prison.
Mr Bout, who is 43, spent more than 15 years allegedly running guns to African warlords and Islamic militants.
He was dubbed the Merchant of Death by a British politician.
But he denies being, or ever having been, an arms dealer - and Moscow also insists he is innocent.
Thai police and airport officials said he had been flown out of Bangkok hours after Prime Minister Abhisit Vejjajiva said the Thai cabinet had backed an appeal court ruling which cleared the way for his extradition.
Dozens of police officers watched over the operation as Mr Bout was transferred from a maximum-security prison to the airport.
His wife, who has been a frequently tearful figure at Mr Bout's court hearings over more than two years of detention since he was arrested, was waiting outside the jail as he was transferred.
But neither she nor the Russian consul were allowed to see him, a Russian embassy official told the BBC.
Mr Abhisit has faced a difficult dilemma over Mr Bout's case, says the BBC's Vaudine England in Bangkok.
A Thai court ruled in August that the extradition should go ahead within three months.
The US even sent a plane to pick him up - but that move proved to be over-confident, our correspondent says.
The courts delayed the extradition again, saying that other charges of money laundering and fraud, earlier laid by US prosecutors, had to be examined.
In October, the court decided to drop those charges, clearing the way for extradition.
But Moscow has been demanding his release, saying that Mr Bout is an innocent businessman and criticising Thai moves to extradite him as politically motivated.
Mr Bout, a former Russian air force officer, is thought to have knowledge of Russia's military and intelligence operations.
Analysts say Moscow is perhaps worried he might turn state witness on trial in the US, and reveal more than Russia would be comfortable with.