Both Anatoly Bilashkov and Vassily Pokchov are to serve 25 years for murdering Salim Khan Yandarbiyev, attempting to murder his 14-year-old son, Dawud, and smuggling arms into Qatar.
Bilashkov had pleaded not guilty to the assassination in February of the retired Chechen official living in exile, but Pokchov admitted responsibility.
In his closing remarks, Judge Ibrahim al-Nisf also concluded there was enough evidence to prove that the assassination had been ordered by Moscow.
Defence lawyer Muhsin al-Suwaidi told Aljazeera.net the pair would be imprisoned despite a prosecution request that the assassins be executed.
Yandarbiyev was president of Chechnya for a brief period after it won de-facto independence from Russia in 1996, later losing out in presidential elections to Aslan Maskhadov.
He was blown up by a car bomb on 13 February in Doha, Qatar, where he had been based since 2000.
Speaking to Aljazeera.net, Chechnya's exiled deputy foreign minister - Ousman Ferzaouli - welcomed the judgment.
"Now the world knows who the real terrorists are. If you remember, British judges found Chechnya's envoy Ahmad Zakayev not guilty of Russian accusations of terrorism. Now Qatar finds Russian agents guilty of murder – a murder with backing from Moscow."
Yandarbiyev's widow also welcomed the judgment. She told Aljazeera.net: "Even though my children will have to remain without a father, I'm pleased that the truth has finally come out. My husband was assassinated by agents acting on orders from Russia."
"I'm pleased that the truth has finally come out. My husband was assassinated by agents acting on orders from Russia"
Salim Khan's widow
However, Russian Foreign Minister Sergei Lavrov insisted that Russia had no connection with the car bombing.
"Moscow's position is still that the two Russian citizens detained in Qatar had nothing to do with the attack on Yandarbiyev."
The closed-door trial began in Doha on 11 April and 13 prosecution witnesses were heard – including a passerby who noted the registration number of a suspicious Russian vehicle on the day of the attack.
The murder sparked a diplomatic row between the Gulf state and Russia.
Yandarbiyev moved to Qatar in
2000, ending his diplomatic career
Qatar expelled the first secretary of the Russian embassy in March after detaining him along with the agents in connection with the car bombing.
Moscow had said it wanted a "friendly" verdict for the two Russians, and there is much talk that the pair will not stay in prison for 25 years.
A Qatari legal expert, Najib al-Nuaimi - who is not involved in the case – believes both Russians could be home within months.
"I expect they will be sentenced as guilty and given either death or a life sentence, then after [Russian President Vladimir] Putin intervenes they are transferred to Russia in a few months," said al-Nuaimi before Tuesday's ruling.
Al-Nuaimi, a former justice minister, noted that Qatar had chosen this path in similar situations before.
Last year, a Jordanian journalist working for state-owned Qatar Television was granted a pardon after he was sentenced to death for charges of spying and passing military and other information about Qatar to Jordan.