The RIA Novosti and Interfax news agencies said Friday the Nerpa submarine manned by an Indian crew has sailed to India.
The sub had an accident that killed 20 Russian seamen during sea trials in 2008. Its fire-extinguishing system activated in error, spewing Freon gas that suffocated the victims and injured 21 others.
Officials have blamed the accident on human error, but details of the official investigation have been kept under wraps.
It was the worst accident in the Russian navy since the sinking of the Kursk nuclear submarine killed 118 seamen in 2000.
Russia has been a top weapons supplier to India since the Soviet times.