Medvedev had won 68.2 per cent of the vote based on results from more than half of the country's polling stations, the election commission said.
None of the other candidates had appeared likely to threaten Medvedev, who had the endorsement of Vladimir Putin, Russia's outgoing president.
On Sunday, Putin congratulated his protege.
"Our candidate, Dmitry Anatolyevich Medvedev, has taken a firm lead," Putin said, wishing him success as he appeared alongside Medvedev on an open-air concert stage in Red Square, outside the Kremlin.
Medvedev, dressed in jeans and a leather jacket, used the occasion to reaffirm his commitment to pursuing Putin's policies.
"Together we can continue the course set by President Putin .... Together we'll go further. Together we'll win. Hurrah!" he declared, to roars of approval from the crowd.
Al Jazeera's Jonah Hull, reporting from Moscow, said the result was "no big surprise"
"The result of this election was never really in any doubt," he said.
"This was a vote in the Soviet style, with really no choice at all."
Even before the election was under way, critics had denounced it as little more than a stage show.
As voting progressed, the independent Russian election monitoring group said it had received a steady stream of complaints.