Moskva, the flagship of Russia's Black Sea Fleet, was being towed to port when "stormy seas" caused it to sink, according to a ministry message.
The 510-crew missile cruiser was a symbol of Russia's military power, leading its naval assault on Ukraine.
Kyiv says its missiles hit the warship. The United States says it also believes it was hit by Ukrainian missiles.
Moscow has not reported any attack - it says the vessel sank after a fire.