Kościuszko was born in the village of Mereczowszczyzna (Belarusian: Мерачоўшчына, Merachoushchyna), now abandoned, near the present-day town of Kosava, Belarus. The area lay within the Grand Duchy of Lithuania, a part of the Polish-Lithuanian Commonwealth.

Kościuszko was the son of a local noble, Ludwik Tadeusz Kościuszko, and Tekla, née Ratomska. He was the youngest child in a family whose lineage traced partly to Lithuanian and Ruthenian nobility^[14] and to a 15th-16th-century courtier of Polish King Sigismund I the Old, Konstanty Fiodorowicz Kostiuszko. At the time of Tadeusz Kościuszko's birth, the family possessed modest holdings in the Grand Duchy. His first language may have been Belarusian, and he was christened in both the Eastern Orthodox and Roman Catholic religions.^[18] As a result of the dual baptisms, he bore the names Andrei and Tadeusz.

In 1765 Poland's King Stanisław August Poniatowski created a Corps of Cadets (Polish: Korpus Kadetów), on the grounds of present-day Warsaw University, to educate military officers and government officials. Kościuszko enrolled on 18 December 1765, becoming a member of the Corps of Cadets. Since the school emphasized both military subjects and the liberal arts, his courses included world history, the history of Poland, philosophy, Latin, the Polish, German and French languages, and law, economics, geography, arithmetic, geometry and engineering. Upon graduation, he was promoted to captain.

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