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. Moscow has not reported any attack – it says the vessel sank after a fire.
The blaze caused the explosion of the warship’s ammunition, Russia says, adding that the entire crew were later evacuated to nearby Russian vessels in the Black Sea.
After saying initially the warship was afloat, late on Thursday the Russian defence ministry announced that the Moskva had been lost.
The 12,490-tonne vessel is the biggest Russian warship to be sunk in action since World War Two.
“While being towed… towards the destined port, the vessel lost its balance due to damage sustained in the hull as fire broke out after ammunition exploded. Given the choppy seas, the vessel sank,” the Russian defence ministry said.