This paper examines the intellectual underpinnings of the nationalism articulated by the followers of Marshal Józef Piłsudski (Piłsudskiites), who ruled Poland between 1926 and 1939. Scholarly consensus holds that modern Polish nationalism was solely the domain of the National Democratic movement. Conversely, the Piłsudskiites' conception of the nation is generally seen as anachronistic, poorly articulated, self-contradictory, and lacking a deeper intellectual foundation. Focusing on the formative years of the Second Polish Republic (1918-1922), this paper draws a link between Piłsudskiite political thought and the philosophy of the heterodox Marxist theorist Stanislaw Brzozowski. Re-examining the early writings of Piłsudski's followers in light of Brzozowski's philosophy, the paper presents the argument that “Piłsudskiite nationalism” was in fact deeply constructivist, surprisingly sophisticated, and no less “modern” than the nationalist discourse articulated by the National Democrats. In the process, the article interrogates and problematizes the classic “ethnic” vs. “civic” typology of nationalist movements.