[1] Founded from the merger of the previously separate towns of Kędzierzyn and Koźle, both dating back to the Middle Ages, the city is a major river port and center of chemical industry, and is particularly known for ZAKSA Kędzierzyn-Koźle, one of the top volleyball clubs in Europe in the 2020s.

A border fortress held by a minor member of the Polish Piast dynasty was first mentioned in 1104, when it was besieged by the Přemyslid prince Svatopluk of Olomouc.

Casimir soon turned to the neighbouring Kingdom of Bohemia; in 1289, he paid homage to King Wenceslaus II and received his duchy as a Bohemian fief.

After Casimir was succeeded by his son Władysław in 1312, Koźle remained the capital of an autonomous duchy, ruled by the Bytom branch of the Silesian Piasts until the death of Duke Bolesław in 1355.

[3] Again purchased by the Opole duke Jan II the Good in 1509, the Koźle estates were ultimately incorporated into the Lands of the Bohemian Crown upon his death in 1532.

The fortress was besieged several times during the Thirty Years' War and occupied by Danish troops under the command of Duke John Ernest I of Saxe-Weimar in 1627, before they were defeated by Imperial forces under Albrecht von Wallenstein.

Occupied by the troops of King Frederick the Great in the First Silesian War, Koźle as Cosel with the bulk of Silesia became a Prussian possession by the 1742 Treaty of Breslau.

The king ordered the extension of the fortifications, nevertheless the town was occupied by Habsburg Pandurs during the Second Silesian War in 1744 and had to be reconquered by the Prussian Army two years later; the shelling again caused heavy losses and damages.

[4] In 1807 the Prussian garrison withstood another besiegement by the allied Napoleonic and Bavarian forces under General Bernhard Erasmus von Deroy until a peace was made by the Treaty of Tilsit.

[5] After World War I and the Upper Silesia plebiscite of March 1921, the Polish insurgents temporarily captured the part of the town east of the Oder during the Third Silesian Uprising.

Kędzierzyn was founded as a village in the 13th century, and Sławięcice was first mentioned in 13th-century documents, when both settlements were part of fragmented Piast-ruled Poland.

[clarification needed] In 1999, the branch line connecting the city with Strzelce Opolskie closed as part of Polskie Koleje Państwowe cost-cutting.