Casimir I the Restorer (b. Kraków, 25 July 1016 – d. Poznań, 28 November 1058), is a Duke of Poland of the Piast dynasty and the de facto monarch of the entire country from 1034 until his death.

He is the only son of Mieszko II Lambert by his wife Richeza, daughter of Count Palatine Ezzo of Lotharingia (of the Ezzonids) and granddaughter of Emperor Otto II.

Casimir is known as the Restorer because he manages to reunite all parts of the Polish Kingdom after a period of turmoil.

He reinstates Masovia, Silesia and Pomerania into his realm.

However, he fails to crown himself King of Poland, mainly because of internal and external threats to his rule.