The Treaty of Trencin was concluded on 24 August 1335 between King Casimir III of Poland and King John of Bohemia together with his son Margrave Charles IV. The agreement was reached by the agency of Casimir's brother-in-law King Charles I of Hungary and signed at Trencin Castle in the Kingdom of Hungary. It initiated the transfer of suzerainty over the former Polish province of Silesia to the Kingdom of Bohemia, whereafter the Duchies of Silesia were incorporated into the Bohemian Crown. Following the integration of this treaty, the three kingdoms of Bohemia, Hungary, and Poland met at the First Congress of Visegrad later in 1335 to further discuss the division of land. This congress also made the treaty official.

Casimir III the Great reigned as the King of Poland from 1333 to 1370. He also later became King of Ruthenia in 1340, and fought to retain the title in the Galicia-Volhynia Wars. He was the last Polish king from the Piast dynasty.