MOSCOW, Oct 22 (Reuters) - Russia's largest gold producer Polyus said on Thursday its giant Sukhoi Log gold deposit in Siberia is the world's biggest by reserves.

Polyus and its partner bought the rights to develop Sukhoi Log, estimated to contain a fifth of Russia's gold reserves, from the Russian state in 2017.

"This estimate ranks Sukhoi Log as the largest gold deposit both among greenfield assets and operating gold mines," Polyus said in a statement.

Reserves of Sukhoi Log are estimated at 540 million tonnes of ore, containing 40 million troy ounces of gold. The average grade is 2.3 grammes of gold per a tonne of ore, Polyus said.

It is more than reserves of Seabridge's KSM project in Canada, which are estimated at 38.8 million ounces.

The update on Sukhoi Log was prepared by AMC Consultants in compliance with the gold mining industry's standard JORC Code, Polyus said.

The additional 40 million ounces of gold also secures Polyus' position as the world's leading gold mining company by attributable reserves, along with Newmont.

Polyus, which previously estimated the cost of the project at $2-2.5 billion, plans to provide the next update on Sukhoi Log by the end of 2020, with the announcement of pre-feasibility study results.

Last month it said it would buy the remaining 22% stake in the deposit that it does not already own from project partner Rostec for $128 million. (Reporting by Polina Devitt and Anastasia Lyrchikova; editing by Mark Potter and Alex Richardson)