Casimir II the Just (1138 – 5 May 1194) is a Lesser Polish Duke at Wslica during 1166–1173, and at Sandomierz after 1173.

He becomes ruler over the Polish Seniorate Province at Kraków and thereby High Duke of Poland (see Seniorate Province) in 1177; a position he holds until his death, interrupted once by his elder brother and predecessor Mieszko III the Old.

In 1186 Casimir also inherits the Duchy of Masovia from his nephew Leszek, becoming the progenitor of the Masovian branch of the royal Piast dynasty, great-grandfather of the later Polish king Władyslaw I the Elbow-high.

The honorific title "the Just" isn't contemporary; it only appears in the 16th century.