Mr Bout has allegedly sold arms to warlords in Africa and Afghanistan
A Thai court has rejected a US request for the extradition of Viktor Bout, an alleged Russian arms dealer dubbed the "Merchant of Death".
The United States wants to put Mr Bout on trial on charges of supplying weapons to Colombian Farc rebels.
"The US charges are not applicable under Thai law," said a Thai judge, who described the US case against Mr Bout as political, not criminal.
Mr Bout was arrested in a Thai-US sting operation in March 2008.
"This is a political case. The Farc is fighting for a political cause and is not a criminal gang. Thailand does not recognise the Farc as a terrorist group," said the judge delivering the verdict at Bangkok's Criminal Court.
The Unites States has 72 hours in which to appeal against the ruling, during which time Mr Bout, 42, will remain in detention.
A decision in the extradition hearing has been repeatedly delayed by a high turnover of defence lawyers.
United Nations agencies and several Western governments have reported that Mr Bout has delivered arms to dictators and warlords in Africa and Afghanistan, allegedly breaking several UN arms embargoes in the process.
He faces US charges of conspiring to kill Americans, conspiring to provide material support to terrorists and conspiring to acquire and use an anti-aircraft missile. He could face a maximum penalty of life in prison if convicted in the US.
His lawyer argues that Mr Bout would not receive a fair trial in the US, where officials say he supplied arms to dictators, al-Qaeda and the Taleban.
He is being held in a maximum security prison outside Bangkok.