It will also allow Rosneft, the world's biggest publicly listed oil firm, to steeply cut its heavy debts and develop new remote Arctic fields.
"The estimate of the sum of the contract in today's market prices is absolutely unprecedented — $270 billion," Russian President Vladimir Putin told an economic forum in Russia's St Petersburg after the deal was agreed.
The agreement highlights a growing partnership between China, the globe's top energy consumer, and Russia, the largest oil producer, and comes despite previously uneasy relations between Rosneft and Beijing over energy pricing.
Rosneft's boss Igor Sechin, a close ally of Putin, said his firm will supply China with 300,000 barrels per day over 25 years starting in the second half of the decade, on top of the 300,000 bpd it already ships to the world's No.2 oil consumer.
Putin later said total supplies could amount to as much as 900,000 bpd.
The speed of change in Russian export patterns has been dramatic — switching huge volumes from Europe in only five years.