Thaddeus C. Radzilowski or Tadeusz Radziłowski (4 February 1938 in Detroit – 20 July 2018) was a Polish-American historian, scholar, author, professor and co-founder of the Piast Institute, a national institute for Polish and Polish-American affairs. Radzilowski's work focused on Poland and other Central and Eastern European nations, including Russia. He wrote extensively on the histories of these regions as well as the migration of peoples from Central and Eastern Europe, with special emphasis on social history and historiography. He lectured widely in Europe and North America and published more than 100 monographs, edited collections, journal articles, book chapters and scholarly papers.