Preliminary results from the Central Election Commission, based on just 9% of the ballots counted, showed United Russia won 38.57% of the vote.
The first results also showed that the Communist Party finished in second place with 25.17% of the vote, followed by the nationalist party LDPR with 9.6%.
Allies of jailed Kremlin critic Alexei Navalny had urged the Russians to follow his tactical voting strategy, which amounts to supporting the candidate most likely to defeat United Russia in a given constituency.
The election was marred by numerous reports of violations, including ballot stuffing, inadequate security and pressure on election observers.
Sky News Moscow correspondent Diana Magnay said it was too early to say whether Mr Navalny’s strategy made a difference.
The big question for United Russia, she added, is “whether they will retain their qualified majority in parliament, or whether they will have to share power a little more”.
But even if it is the latter, “the sharing of power is done anyway with parties that are mainly managed by the Kremlin,” she stressed.
Initial results showed that three parties that almost always vote for Kremlin initiatives were likely to return to the State Duma – the lower house of parliament.
United Russia currently has a two-thirds majority, which allows it to change the constitution.