Kazimierz I the Restorer (Polish: Kazimierz I Odnowiciel; English: Casimir) (26 July 1016 - 28 November 1058), was a Duke of Poland of the Piast dynasty and the de facto monarch of the entire country. He is known as the Restorer mostly because he managed to reunite all parts of Poland after a period of turmoil and permanently attached Masovia, Silesia and Pomerania. Son of Mieszko II Lambert and Richensa of Lotharingia, Casimir failed to crown himself the King of Poland, mainly because of internal and external threats to his rule.

Relatively little is known of Casimir's early life. Born to Mieszko II of Poland and Richensa of Lotharingia, he must have spent his childhood at the royal court of Poland in Gniezno. Mieszko II was crowned the king of Poland in 1025 after his father's death. The many landlords, however, feared the single rule of the monarch. This situation led to conflicts in the country, in which Mieszko's brothers turned against him and the Emperor Conrad II's forces attacked the country, seizing Lusatia. Years of chaos and conflict followed, during which Mieszko died (1034) in suspicious circumstances after his forced abdication and a brief restoration.

After the death of her husband, Richensa probably tried to seize the power in the country and secure the crown for her son. However, she failed and Casimir had to flee to the Kingdom of Hungary while the central parts of Poland were controlled by Bezprym. The region of Greater Poland revolted against the nobles and clergy and a mass Pagan revival ensued there. Also the land of Masovia seceded and a local landlord named Mieclaw formed a state of his own there. Similar situation happened in Pomerania, where the power was held by a local dynasty loosely related to the Piasts. Duke Bretislaus I of Bohemia, observing the period of turmoil in Poland, took advantage of his neighbour's weakness and invaded the country. After a short struggle he conquered Silesia and Lesser Poland and severely pillaged Greater Poland, burning Gniezno to the ground and looting the relics of Saint Adalbert.

The following year the new Holy Roman Emperor, Henry III, allied himself with the exiled Polish ruler against the Bohemians. Casimir was given a troop of 1,000 heavy footmen and a significant amount of gold to restore his power in the country. Casimir also signed an alliance with Yaroslav I the Wise, the Prince of Kievan Rus'. The alliance was sealed by Casimir's marriage with Yaroslav's sister, Maria Dobronega. With such support Casimir returned to Poland and managed to retake most of his domain. In 1041 defeated Bretislaus signed a treaty at Regensburg in which he renounced his claims to all Polish lands except for Silesia, which was to be incorporated into the Crown of Bohemia. It was Casimir's success in strengthening royal power and ending internal strife that earned him the epithet of "the Restorer".

The treaty gained Casimir a period of peace at the southern border and the capital of Poland was moved to Kraków, the only major Polish city relatively untouched by the wars. It is probable that the Holy Roman Emperor was happy with the balance of power restored in the region and forced Casimir not to crown himself the king of Poland. In 1046 Emperor Henry held royal and imperial courts at Merseburg and Meißen, at which he ended the strife among the Dux Bomeraniorum (Duke of Pomerania), Duke Bretislaus of Bohemia, and Poland's Casimir I. In 1047 Casimir, aided by his Kievan ally, started a war against Masovia and seized the land. It is probable that he also defeated Mieclaw's allies from Pomerania and attached Gdansk to Poland. This secured his power in central Poland. Three years later, against the will of the emperor, Casimir seized Czech-controlled Silesia, thus securing most of his father's domain. In 1054 in Quedlinburg the Emperor ruled that Silesia was to remain in Poland in exchange for a yearly tribute of 117 kilograms of silver and 7 kg of gold.

At that time Casimir focused on internal matters. Conflicted with the Emperor in the Silesian case, he supported the Papacy in the Investiture Controversy and gained the support of the church. To strengthen his rule he re-created the bishopric in Kraków and Wroclaw and erected the new Wawel Cathedral. During Casimir's rule heraldry was introduced in Poland and, unlike his predecessors, he promoted landed gentry over the druzyna as his base of power. One of his reforms was the introduction, to Poland, of a key element of feudalism: the granting of fiefdoms to his retinue of warriors, thus gradually transforming them into medieval knights.