At least 14 sailors aboard the Russian submarine died in the fire due to smoke inhalation.
The Defence Ministry stated that the blaze had erupted on Monday while the deep-sea military submersible, designed to research the bottom of the sea, was running biometric tests in Russian territorial waters.
"Thanks to the selfless actions of the team, the fire was put out", the ministry said in a statement.
The Russian Investigative Committee has said it had launched an investigation into the deadly fire.
At the moment, the deep-sea vehicle is placed in Severomorsk, which is the main administrative base of the Russian Northern Fleet.