Ekho Moskvy quoted Aleksanyan's lawyer, Yelena Lvova, as saying that the operation was performed on Friday. It was not immediately clear why his spleen had to be removed.
Aleksanyan, 36, diagnosed with AIDS and cancer, is accused of embezzling about $330 million from former Yukos production unit Tomskneft and shares worth $493 million from other oil companies, as well as laundering stolen assets.
In early March, the Moscow City Court ruled that it was lawful to provide hospital treatment for Aleksanyan while under guard.
The prosecution earlier insisted Aleksanyan undergo medical treatment in jail. However, a Moscow court suspended the trial due to his deteriorating health.
Jailed ex-Yukos CEO Khodorkovsky, the founder of what was once Russia's largest oil producer, declared a hunger strike on January 30 demanding medical treatment for Aleksanyan. He called off his protest some two weeks later when Aleksanyan was admitted to hospital.
Aleksanyan earlier claimed the reluctance of Russian authorities to provide him with medical treatment was due to his refusal to assist prosecutors with new charges against Khodorkovsky.
Yukos collapsed after claims of tax evasion in 2004 which led to the company being broken up and sold off to meet creditor claims. The bulk of its assets were bought through liquidation auctions by government-controlled oil company Rosneft. Yukos was officially terminated in 2007.
