Leon Wasilewski (1870–1936) was an activist of the Polish Socialist Party (PPS), a coworker of Józef Piłsudski, Polish Minister of Foreign Affairs, designer of much of Second Polish Republic policy towards the East, historian and father of Wanda Wasilewska.

Born on 24 August 1870 in St. Petersburg, to a family of Polish émigrés from Livonia, his education stopped at the level of gymnasium. After a brief participation in the endecja movement (Liga Narodowa), he joined the PPS in Galicia in the 1890s - he would remain a member of the party for his entire life (and join the party's top policy-making body in the late 1920s). Editor of Przedświt (PPS publication in London), and later, Robotnik in Galicia. He was loyal to Piłsudski, even during the PPS split in the 1900s, when he remained with his Revolution Faction. Activist in the Komisja Tymczasowa Skonfederowanych Stronnictw Niepodległościowych (1912-1914). During the First World War, he was a member of several Polish organizations, including the Chief National Committee, and from 1917, a member of the Polish Military Organisation. After Poland regained independence, he became the first Polish Minister of Foreign Affairs, serving in the government of Jędrzej Moraczewski from 17 November 1918 to 16 January 1919. Afterwards he served as an advisior to naczelnik państwa, Józef Piłsudski; he was a member of the Polish National Committee in Paris (in 1919); and served as the Polish ambassador to Estonia (1920-1921). He took part in the Treaty of Riga negotiations and the commission for the delimitation of Poland's eastern borders.

Afterwards he concentrated on historical research. He researched linguistics (particularly Slavic languages), ethnography of the Central and Eastern European lands, and history of literature. He would serve as the director of two institutes (Instytut Badania Najnowszej Historii Polski (Institute of Studies of Modern Polish History) in the 1920s and Instytut Badań Narodowościowych (Institute of Nationality Studies) in the 1930s) and editor of the journal Niepodległośc. Supporter of Międzymorze federation idea, as well as Prometheism, he was also a vocal opponent of polonization, arguing that Ukrainians and Belarusians living in Poland should be allowed to assimilate into Polish society at their will and speed. Author of many works, among them Litwa i Białoruś ("Lithuania and Belorussia", 1912), Ukraińska sprawa narodowa w jej rozwoju historycznym ("The Ukrainian National Cause in its Historical Development", 1925), Zarys dziejów PPS ("A Short History of the PPS", 1925), Józef Piłsudski, jakim go znałem ("Józef Piłsudski, as I knew him", 1935).

His daughter, Wanda Wasilewska, was a prominent pro-Soviet communist activist in the People's Republic of Poland.