The Soviet Union has granted Yelena Bonner, wife of dissident Soviet physicist Andrei Sakharov, permission to remain in the West for an additional three months, an authoritative Soviet source said here today.

Bonner, who left the Soviet Union in early December for medical treatment in the West, is recuperating after heart surgery last month in a Boston hospital. She is staying with her mother, son and daughter in Newton, Mass.

Bonner's visa to travel to the West for eye treatment and heart surgery, granted last November for three months, has been extended for three more months, according to Victor Louis, a well-connected Soviet journalist who has been an occasional source of information about Sakharov and Bonner.

The West German daily Bild reported today that Soviet authorities had granted Bonner an extension of her visa, which expires Feb. 28. Louis said such medical extensions are granted "pro forma."

Bonner's relatives in Newton indicated two weeks ago that she was seeking the visa extension. The 62-year-old Bonner left the closed Soviet city of Gorki -- where she and Sakharov have been living in internal exile -- at the end of November and flew to Rome Dec. 2 for eye treatment before going to Boston.

[Bonner's mother said from Newton that she had no knowledge of the visa extension. Other family members were not available.]

Before leaving Moscow Bonner had said that her return was contingent on an agreement not to hold interviews or press conferences with western reporters. Last week, Bonner appeared at a news conference on Soviet human rights at the United Nations, but she made no statements.

Louis said that nothing Bonner has done during her stay in the West would prevent her return to the Soviet Union.

Her visa to leave, granted shortly before Soviet leader Mikhail Gorbachev and President Reagan met in Geneva, was widely seen as a concession to ease East-West tensions.