Lajos Pósa (Budapest, December 9, 1947) is a Hungarian mathematician, working in combinatorics. Paul Erdős's favorite "child", he discovered theorems at the age of 16. Since 2002 he works at the Rényi Institute of the Hungarian Academy of Sciences; earlier he was at the Eötvös Loránd University, at the Departments of Analysis, Computer Science. He received the Károly Simonyi Research Scholarship in 2000.
Pósa is one of the most active and most successful mathematical educators in Hungary. He taught many of the famous Hungarian mathematicians (László Babai, Imre Z. Ruzsa, Gábor Tardos). Most of his students, however, do not become professional mathematicians. He organizes special 1–2 day long mathematical camps for carefully selected 12–18 year old children, one group containing 25–35 kids meeting twice a year. He does not aim to teach hard, established parts of mathematics, but rather makes the participants discover mathematical results via solving carefully chosen elementary problems in groups of 3 or 4.