Castro's eight-day visit is part of a push by Moscow to strengthen its own influence, and counter United States influence, in South America.
Medvedev visited Cuba and other South American nations in November, and Russia also sent a navy squadron to the Caribbean.
At Zavidovo on Thursday, Medvedev helped wrap a coat around Castro's shoulders when he emerged from a car.
The pair exchanged pleasantries in front of a fireplace before heading out for a stroll in the wintry woods and sharing dinner.
Medvedev said Castro, the 77-year-old brother of Fidel Castro, the former Cuban leader, had been to the same spot nearly 25 years ago, and Castro recalled eating salo (pork fat) roasted over an open fire in the forest.
The Russian president said: "We will of course eat salo with black bread and we will talk about everything of interest to us. I also have a film about Commandante Fidel Castro's stay, and there are interesting photos here.''
"Of course I was younger then," said Castro.
Before the Soviet Union collapsed in 1991, it provided billions of dollars in trade and subsidies to Cuba.
Both countries are said to be looking out for changes in the foreign policy of the US, under the leadership of Barack Obama, the country's new president.
Washington currently maintains an economic blockade of Cuba.
"Our country has consistently stood for normalising the situation around Cuba, for its fully fledged reintegration into regional and world processes," said Sergei Lavrov, the Russian foreign minister.
"What is important is that Latin American countries are united in their efforts to overcome Havana's isolation."