Vilnius Regional Court handed down the minimum sentence as Osurkov had no previous convictions and pleaded guilty. In Lithuania, people face up to 15 years in prison for spying.
According to the court, while serving in the Lithuanian army, Osurkov collected information on Lithuania's military equipment, training programmes, military facilities and troops' loyalty to the state. He would later hand it to a Belarusian intelligence officer.
Osurkov was under surveillance from August 2012 until January 2014 when he was arrested.
Prosecutors had asked the court to sentence him to six years in prison. Prosecutor Redas Saviskas did not say if he would appeal the verdict. The defendant's lawyer refused to speak to journalists on Friday.
It was established during the hearing that Osurkov lived and worked in Minsk, Belarus, in 2006 and there he met with Sergey Kurulenko, head of the Belarusian intelligence organization, agreeing that he would return to Lithuania, join the army and provide information to Belarus about Lithuania's army in exchange for money.
The defendant told the court that he had no intention to do damage to the State of Lithuania and only wanted to make some money.
In a separate case, Romuald Lipskij, a former employee of state company Oro Navigacija, was also sentenced to three years and three months in prison in November for spying for Belarus.