"I voted for the young, who are having a hard time now, but who are the future," Yeltsin told Russia's Interfax news agency.
"All citizens of Russia should surely vote," said the former Communist leader, who is credited with dismantling the Soviet Union and installing democracy.
Opinion polls suggest United Russia, a party that supports President Vladimir Putin, was expected to win a workable majority, with the Communist party trailing in second place.
The election was declared valid shortly after midday Sunday by Alexander Veshnyakov, head of the Central Election Commission, who said more than 30 percent of Russians had already voted. At least 25 percent of Russia's 110 million registered voters have to cast ballots before an election is considered valid.