Martin Michael Sellner, born on January 8, 1989, in Wels, Upper Austria, is an Austrian far-right extremist political activist and a prominent figure in the Identitarian movement, known for promoting white nationalist, anti-immigration ideologies and the 'Great Replacement' conspiracy theory across Europe. He grew up in a middle-class family and showed early interest in right-wing ideologies, influenced by online communities and nationalist literature. Sellner briefly studied philosophy at the University of Vienna starting around 2007 but dropped out without completing a degree to focus on political activism. His early involvement included participation in neo-Nazi groups and skinhead scenes during his teenage years, though he later distanced himself from overt neo-Nazism to adopt a more 'intellectual' far-right persona under the Identitarian banner.

In 2012, Sellner co-founded and leads the Identitarian Movement of Austria (Identitäre Bewegung Österreich), which promotes an anti-immigration, ethno-nationalist agenda inspired by the French Identitarian group Génération Identitaire. As its leader, he has organized provocative campaigns such as 'Remigration' initiatives calling for the deportation of non-white immigrants and protests against asylum seekers. Sellner is a key proponent of the 'Great Replacement' conspiracy theory, authoring books and running a blog and YouTube channel to spread these ideas. He has connections to international white nationalist networks, including consultations for foreign far-right figures like Lutz Bachmann of PEGIDA in Germany, and has been linked to the alt-right through events like speaking at American Renaissance conferences. Sellner attempted to enter mainstream politics by running for the Freedom Party of Austria (FPÖ) in 2019 but was unsuccessful.

Sellner's activities have drawn significant controversy and legal scrutiny. He has been banned from entering several countries, including the UK in 2018, Australia in 2019, and the US in 2020, due to his extremist views and suspected involvement in planning violent acts, such as a 2018 donation from the Christchurch mosque shooter Brenton Tarrant. In Austria, he faced investigations for hate speech and was fined in 2021 for inciting hatred online. Critics, including anti-fascist groups and governments, have condemned him as a dangerous ideologue fostering xenophobia and white supremacy, with allegations of ties to violent extremists persisting despite his claims of non-violence.