The deal, to be done by Russia's biggest oil company Rosneft, will be worth $85 billion, Medvedev said.
"It is a big sum of money to any country, even to China. It testifies to the fact that we have reached a higher and a brand new level of cooperation," he said.
China's oil imports stood at 276 million tons last year, with Russia being the fourth-largest provider.
Medvedev also said he hopes that a contract on natural gas deliveries to China will be signed soon.
Under a memorandum of understanding signed in March, Russia will supply 38 billion cubic meters of gas to China annually starting in 2018. The volume is expected to increase to 60 billion cubic meters in following years.
Shortly before the online chat, Premier Li Keqiang and Medvedev co-chaired the 18th regular meeting of the Chinese and Russian heads of government.
The regular meeting has become an important platform to outline cooperation between the two countries.
"Energy cooperation between China and a third party will not affect cooperation between China and Russia," Li said, adding that China-Russia energy cooperation is of long-term strategic significance.
Cooperation between China and Russia is also collaboration between major emerging economies, Li said.
"The bilateral ties are of great significance to Eurasia and the whole world ... and will benefit world economic recovery," he added.
Medvedev said there is potential for Sino-Russian trade given that the countries weathered the global recession and managed to reverse the trade downturn.
Xing Guangcheng, a researcher of Russian studies at the Chinese Academy of Social Sciences, said oil and gas imports from Russia should be China's first choice as Russia is a major energy exporter and is close to China.
