Ivan Sleshinsky or Jan Śleszyński (Russian: Иван Владиславович Слешинский) (1854–1931) was a Polish-Russian mathematician. He was born in Lysianka, Russian Empire to Polish parents.
Śleszyński's main work was on continued fractions, least squares and axiomatic proof theory based on mathematical logic. He and Alfred Pringsheim, working separately, proved what is now called the Śleszyński–Pringsheim theorem.
His most important publications include: "Teoria dowodu" ("The theory of proof") in two volumes (1925, 1929), and "Teoria wyznaczników" ("The theory of determinants") (1926).