Marius Sebastian Ostrowski FRHistS FRSA (German: [ˈmaːʁɪʊs zɛˈbastɪan ɔsˈtʁɔvskiː]; born 12 December 1988) is a German-British political and social theorist, historian of ideas, policy researcher, and composer, based at the Blavatnik School of Government, University of Oxford.

He also writes on the theory and history of social democracy, in particular its origins in interwar socialist reformist thought, and on progressive visions of European integration, including the prospects for a Europe-wide Universal Basic Income.

His mother was the Romanian-German-British actress Doris Hermann-Ostrowski, née Rodica-Doris Bogdan (1952–2023), who later became a lecturer and tutor in German language studies at the Goethe Institut, London School of Economics, and University of Oxford.

[14] In 2019, a Visiting Fellowship at the European University Institute (EUI) in Florence introduced Ostrowski to the neglected Europeanist ideas of interwar progressive thinkers such as Max Cohen-Reuß and Alexandre Marc.

[15] As a result, in 2020 he moved to the EUI as a Max Weber Fellow at the Robert Schuman Centre for Advanced Studies to pursue a project on the ideological history of Europeanism in the 20th century.

[24] Ostrowski took up an offer to join the thinktank ResPublica in 2022 to develop a new strand of research on lifelong learning, integrated tertiary education, and the political economy of skills.

[28] Ostrowski began to learn piano at the age of 3, followed soon after by the clarinet, organ, and music composition, studying with Ruth Nye, Stephen Goss, and James Weeks.