Vladimir Veitman, a former specialist for Estonian counterintelligence, has been found guilty of spying for Russian secret services.
Officials on Wednesday also revealed the name of his recruiter and said Veitman had warned Russian services that Estonian authorities were on to another agent.
In a closed courtroom, Veitman was sentenced to 15 years in prison for treason and other charges in a plea bargain.
Having retired from the Estonian counterintelligence and anti-corruption agency KaPo (often referred to in English as the Internal Security Service) in 2011, Veitman was arrested last August over suspicions that he had provided classified Estonian intelligence to Russia's secret services over the years. He pled guilty and was said to be cooperative with investigators.