Sanseitō (Japanese: 参政党, lit. 'Political Participation Party'; self-rendered as Party of Do It Yourself in English, DIY) is an ultraconservative, right-wing populist political party in Japan. It is on the far-right of the political spectrum. The party was founded in 2020 by Sohei Kamiya, the current secretary general of the party, and won a seat in the 2022 House of Councillors election, also becoming an official political party by winning more than 2% of the vote in the election. Sanseitō gained international media attention during the 2022 House of Councillors election due to Kamiya's antisemitic rhetoric during public appearances and campaign rallies. In the 2024 general election, the party won 3 seats. In the 2025 House of Councillors election, the party won more than 14 seats, bringing its total seat number to 15. According to the party's leader Kamiya, it is the Japanese equivalent of Trumpism in the United States.

The party promotes COVID-19 misinformation. The party's president, Manabu Matsuda, has called COVID-19 vaccines a "murder weapon". The party is against same-sex marriage and LGBT rights. The party strongly opposes immigration, claiming that foreigners bring crime and receive better treatment than native citizens. It proposes the creation of a new constitution to replace the existing one and published a draft that contains minimal human rights protections.

Sanseitō originated from a conservative YouTube channel called "Political Party DIY" created by streamer Kazuya Kyoumoto [ja], politician Sohei Kamiya, and political analyst Yūya Watase [ja] in April 2019. The YouTube channel's objective is to show how to create a political party from scratch. In April 2020, the three founders of the YouTube channel officially started Sanseitō.

At the end of 2020, Jōichiro Shinohara [ja], a founding member of the party, advocated a conspiracy theory alleging that the 2020 United States presidential election was "rigged", causing infighting among him and two founding members, Kyoumoto and Watase, who were skeptical of the conspiracy theory. This led to three of the founding members leaving the party in 2021, while Kamiya remained as the leader of the party with Manabu Matsuda, another founding member of the five-person party. At this time, Kamiya was thinking about the direction of the party, and decided that he would be able to expand his support base by focusing more on conspiracy theories and network marketing. Since then, a large number of anti-vaccination and organic faith groups who claim that the COVID-19 vaccine is a conspiracy by pharmaceutical companies joined the party, creating a new support base.

The party fielded five candidates for the national proportional representation block and 45 candidates in all constituencies for the 2022 Japanese House of Councillors election. Sohei Kamiya, a Sanseitō candidate in the national proportional representation block, won a seat. The party received more than 2% of the vote in the constituencies and proportional representation block, meeting the legal requirements for it to become a political party.

In the 2025 House of Councillors election, Sanseitō secured 14 seats based on its "Japanese First" slogan and a policy of tax reduction. It has gained votes from voters who had no previous party affiliation. However, its policy direction, which can be viewed as discriminatory, remained a contentious point for a party that aims to be influential in national politics in Japan.

Sanseitō is known for its ultraconservative ideology. It has also been referred to as extreme, "extremely conservative", and "hardline nationalist".

Sohei Kamiya, founder and secretary general of Sanseitō, said that Sanseitō has a high affinity with the conservative faction of the U.S. Republican Party, the far-right German party Alternative for Germany (AfD), the far-right French party National Rally (RN), and the right-wing populist party Reform UK. Kamiya has said that he learned many of his "emotional button-pushing themes and norm-breaking language" from U.S. President Donald Trump and stated that he was Japan's equivalent of Trump. According to Kamiya, Japan faces threats from shady globalists ("cabal of global elites"), criminal foreigners and a corrupt political establishment that is suffocating young people with taxes. His has proposed a "Japanese First" nationalist agenda. Sanseito has drawn mainly young male voters. Opponents and domestic media reports have accused him of being xenophobic, saying he is directing public anger with high prices and stagnant wages at Japan's foreign residents.