Exit polls indicated that four other Kremlin-loyal parties will also enter the 450-seat State Duma: the Communists, the Liberal Democrats, A Just Russia and New People.
Turnout was 56.66% at 8.55pm Moscow time. Voting began Friday.
No opposition party was allowed to run. In August, the Supreme Court rejected an appeal by the anti-war Yabloko party against its exclusion, although polls suggested it could clear the 5% threshold.
Yabloko leader Nikolai Rybakov spoiled his ballot, writing the party's name and the slogan "For peace" on it.
Exiled opposition figures Yulia Navalnaya and Maxim Kats urged voters to back any party except United Russia. In single-seat districts, they urged support for the strongest candidate, whose "high result would be unpleasant for the Kremlin and United Russia".
The Kremlin cast the vote as a show of support for soldiers fighting in Ukraine, and many veterans of the war stood for United Russia and loyalist parties.
President Vladimir Putin said Wednesday that voting would help secure victory and demonstrate national resilience against external pressure.
The war reached the polling days directly. Ukraine launched its largest drone attack on the Moscow region on Sunday, killing three people and hitting the capital's oil refinery, one of Russia's largest.
Moscow Mayor Sergei Sobyanin called it an "unprecedented attack" meant to disrupt the vote.
Two people, including an election official, were also killed by a drone in the Russian-controlled part of Ukraine's Kherson region.
In Moscow, a failure of the electronic voter-registration system temporarily halted the issuing of paper ballots. Authorities had reported numerous attacks on election systems and communication lines on Saturday but said the situation was under control.
European Commission spokesman Christian Wigand said Friday that the conduct of the election "shows the erosion of democracy in Russia" and that the Kremlin "deprived voters of a genuine choice".
Ukraine's foreign ministry condemned voting in occupied territories. "No 'voting' organised by Russia at gunpoint changes the internationally recognised status of these territories", it said.
Poland's foreign ministry also condemned the votes there, as well as in Transnistria, South Ossetia and Abkhazia, saying they have no legal effect.
Ukrainian military intelligence says Russian authorities plan a mobilisation after the vote. President Volodymyr Zelensky said in late August, citing the agency, that Moscow wants to conscript several hundred thousand people by the end of the year, and more in a second wave in 2027.
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Source: PAP