Legislative elections were held in Russia from 17 to 19 September 2021. At stake were 450 seats in the 8th convocation of the State Duma, the lower house of the Federal Assembly. Going into the elections, United Russia was the ruling party after winning the 2016 elections with 343 of the 450 seats and retaining a supermajority. In March 2020, it was proposed to hold a snap election in September 2020 due to proposed constitutional reforms, but this idea was abandoned. On 18 June 2021, Vladimir Putin signed a decree calling the election for 19 September the same year. Owing to the COVID-19 pandemic in Russia, voting in the election lasted for three days, from 17 to 19 September. Final turnout was reported to be 51.72%.

Fifteen political parties applied for participation, 14 of which were guaranteed automatic access to the ballots, and one unsuccessfully attempted to be included in the ballot by collecting voters' signatures. Half, or 225 seats, of the State Duma were elected through legislative constituencies; the other 225 seats were elected through party lists, which cover the whole of Russia.

Like prior elections in Russia, the election was not free and fair. Many episodes of ballot-stuffing, forced voting, and other irregularities were recorded. Putin's administration and the ruling United Russia party used a managed democracy approach to keep an appearance of political pluralism. The election was marred by the nation's most prominent opposition leaders (particularly those associated with jailed opposition leader Alexei Navalny) and figures being excluded from the ballot, imprisoned or exiled in months coming before the election. Authorities also designated various independent media outlets and non-governmental organisations as "foreign agents", including the independent election monitor Golos. The Organization for Security and Co-operation in Europe (OSCE) also said that it would not send observers for the first time since 1993 due to "major limitations" imposed by Russian authorities.

Following the election, United Russia maintained its constitutional majority despite some losses, taking 324 seats, while it received 49.82% of the vote, according to official results. The Communist Party of the Russian Federation received 57 seats and 18.93% of the vote, while A Just Russia — For Truth received 27 seats and 7.46% of the vote, with the Liberal Democratic Party of Russia receiving 21 seats and 7.55% of the vote. New People received 13 seats and 5.32% of the vote, meaning that for the first time since 2007, a fifth faction would be represented in the State Duma.

The election was marred by widespread accusations of fraud. The introduction of remote electronic voting in several regions was also criticized by the opposition, who alleged widespread vote-rigging, especially in Moscow, with the Communist Party refusing to recognize the results of electronic voting in Moscow. As a result, opposition candidates have sought to annul the electronic voting results by legal means. In response to the election results, protests were held. Statisticians have attributed the results to mass fraud.

After the 2016 Russian legislative election, United Russia was the most popular party, with its polling rates ranging from 40% to 55%, while the ratings of the main opposition parties were much lower; the Communist Party of the Russian Federation (CPRF) and the Liberal Democratic Party of Russia (LDPR) each had just over 10%, while A Just Russia (JR) was around the 5% threshold required to enter the State Duma.

In June 2018, after Prime Minister Dmitry Medvedev announced a reform to raise the retirement age, the rating of ruling party United Russia fell sharply and since then has fluctuated from around 35% to 25% sparking the 2018 Russian pension protests during summer. At the same time, the CPRF's rating has grown and currently ranges from 11% to 17%, while the rating of JR had also increased but still remained relatively low and varied in between 5% and 8%, and the rating of the LDPR has remained consistent at around 10% to 13%. Polling results differ on whether the CPRF or the LDPRF are ranked as the second most popular party. The ratings of all non-parliamentary parties combined ranges from around 6% to 17%, with exact numbers for each party unknown. When the share of people who answered they would not vote and who are unsure are equally distributed among the parties, voter share for United Russia fluctuates around 41%, the CPRF around 20%, the LDPR around 15%, JR around 8%, and other parties around 14%.

During his 2020 Presidential Address to the Federal Assembly on 15 January, President Vladimir Putin proposed a referendum to be held on the number of amendments arguing that a national vote would be necessary to make the changes to the Constitution of Russia "legitimate". The amendments proposed to weaken the executive power and expand the powers of the Federal Assembly. In particular, it is supposed to transfer the power to form a government to the State Duma; this means that if the amendments were adopted, the next Cabinet would be formed by the State Duma. Immediately after Putin's speech, Prime Minister Dmitry Medvedev resigned along with the entire government. On 16 January, Mikhail Mishustin was appointed as the new Prime Minister. After that, there were suggestions about the dissolution of the State Duma and holding a snap election after the constitutional reform; the State Duma can only be dissolved if it refuses the President's appointment of a Prime Minister three times in a row or passes a no-confidence motion against the government. According to sources from the Presidential Administration of Russia, the constitution may be amended to allow self-dissolution of the State Duma. Some political scientists and MPs also have said that the dissolution of the State Duma and holding of a snap election is likely; however, this opinion is not shared by Andrey Klishas, head of the Legal Committee of the Federation Council, who is part of the working group on preparing amendments to the constitution.