Nathan Birnbaum (Hebrew: נתן בירנבוים; pseudonyms: "Mathias Acher", "Dr. N. Birner", "Mathias Palme", "Anton Skart", "Theodor Schwarz", and "Pantarhei"; 16 May 1864 – 2 April 1937) was an Austrian writer and journalist, Jewish thinker and nationalist.[1][2] His life had three main phases, representing a progression in his thinking: a Zionist phase (c. 1883 – c. 1900); a Jewish cultural autonomy phase (c. 1900 – c. 1914), which included the promotion of the Yiddish language; and a religious phase (c. 1914–1937), when he turned to Orthodox Judaism and became staunchly anti-Zionist.

He married Rosa Korngut (1869–1934) and they had three sons: Solomon (Salomo) (1891–1989), Menachem (1893–1944), and Uriel (1894–1956).