According to a report from Russian state-run news agency Tass, United Russia won 54.28% of the vote, with 93% of all votes counted.
Tass says preliminary results from the Central Election Commission suggests the party will win 343 seats of the 450 on offer.
The Communist Party is expected to win 42 seats with 13.54% of the votes, ahead of the Liberal Democratic Party on 39 seats and the Fair Russia Party on 23.
Turnout for the State Duma vote was 47%, according to Tass -- well down on the 60% five years ago.
Head of the CEC Ella Pamfilova told reporters Monday that she did not expect any significant changes in the results.
"Of course, there can be some minor changes in the results, but on the whole the CEC believes that no major changes are expected. In fact, the results are rather clear and definitive," she was quoted as saying by Tass.
"I hope these results properly reflect reality and if someone has any doubts they can come to us, they are welcome.
"I am sure that most members of the Commission didn't expect such a result, it came as a surprise to us in many ways."
The Putin factor
The result signals an improvement for United Russia, which won 49% of the vote and 238 seats during the 2011 election.
A gain of 105 seats means United Russia has inflicted heavy losses on its opponents.
The Communists have lost 50 seats, Liberal Democratic Party have lost 17, and A Just Russia lost 41.
The party has benefited hugely from its association with Putin, who has spent the past 17 years as either Prime Minister or President.
Putin's next target is the presidential election in 2018.