The Venezuelan president's trip, which is expected to focus on military co-operation and trade, is his third visit to Russia since June 2007.
It comes just three days after Russia sent a massive naval fleet to the Caribbean for joint exercises with Venezuela.
Moscow has been stepping up its relations with Venezuela in a pointed response to Washington's support for Georgia and other aspects of what the Kremlin sees as United States encroachment near its borders.
According to Russian media reports, Venezuela has been asking for an arms loan from Moscow for months.
Chavez's latest world tour is taking him to several countries whose governments are eager to limit the influence of the US.
His trip to Russia followed talks in China, with Hu Jintao, the country's president.
Russia's Kommersant daily newspaper reported on Thursday that the visit was part of the run-up to local elections in Venezuela and that Chavez would use the trip to stress his alliance with Russia against the US.
Chavez and Medvedev are expected to observe Russian army exercises taking place in Orenburg, Kommersant reported, an event that would emphasise the growing military ties and arms sales between the two countries.
Earlier this month, in deployments not seen since the Cold War, Russia sent two long-range bombers to Venezuela for exercises and has dispatched a flotilla of warships from the Arctic base of Severomorsk to Venezuela near US waters.
Venezuela has bought Russian fighter jets, tanks and assault rifles and is planning to purchase anti-aircraft systems, armoured personnel carriers and more combat aircraft, Kommersant reported earlier, quoting Russian officials.
During his visit to Russia, Chavez is also expected to make a statement supporting Russian military action against Georgia last month.
US-Russia relations are at one of their lowest points since the end of the Cold War because of tensions over Russia's conflict with Georgia and US plans to site missile defence facilities near Russia's borders.
Venezuela, which is to hold local elections in November, expelled the US ambassador to Caracas earlier this month in what Chavez said was an act of solidarity with Bolivia which also expelled its US envoy.
After his visit to Russia, Chavez is due to visit France and Portugal as part of an international tour that has already taken him to China and Cuba.
Neave Barker, Al Jazeera's correspondent in Moscow, said: "The relationship between Venezuela and Russia has been good now for a number of years.
"But the [arms deal] does appear to time specifically to the wake of that war between Russia and Georgia. A pointed attempt perhaps ... to send a clear signal to the United States against their support for Georgia."
Meanwhile, Medvedev said on Thursday that Russia's plans to modernise its armed forces will not be affected by the global financial crisis.
"Regardless of any crisis we should build new submarines, should simply deal with the modernisation of the armed forces," Medvedev told the crew of St George the Victor nuclear submarine during a visit to its Pacific home base at Kamchatka Peninsula.
"Our country has means and resources for that," he said in comments showed on national television.
Putin made the revival of the armed forces, neglected in the first post-Soviet decade, a symbol of Russia's resurgence and an additional argument in Moscow's foreign policy.
Medvedev has said the recent war in Georgia showed Russia needed to equip its army with more advanced weaponry and Putin has said the military budget will grow by 28 per cent next year.