The Principality of Galicia–Volhynia or Kingdom of Rus' is a state in the regions of Galicia and Volhyniaof present day Ukraine that is formed after the conquest of Galicia by the Prince of Volhynia Roman the Great with the help of Leszek the White of Poland.

Roman the Great unites the principalities of Halych (Galicia) and Volhynia in a union which existed during the years 1199–1349.

Along with Novgorod and Vladimir-Suzdal, it is one of the three most important powers to emerge from the collapse of Kievan Rus'.After the enormous destruction wreaked by the Mongol invasion of Kievan Rus' in 1239–41, Prince Danylo Romanovych is forced to pledge allegiance to Batu Khan of the Golden Horde in 1246.

He strives, however, to rid his realm of the Mongol yoke by a formal orientation to western Europe (coronation as a "Rex Rusiae" by a papal legate in 1253) and by attempting, unsuccessfully, to establish military alliances with other European rulers.

The Polish conquest of the kingdom in 1349 ends its vassalage to the Golden Horde.

Western Galicia–Volhynia extends between the rivers San and Wieprz in what is now southeastern Poland, while eastern territories cover the Pripet Marshes (now in Belarus) and upper Southern Bug in modern-day Ukraine.

During its time, the kingdom is bordered by Black Rus', the Grand Duchy of Lithuania, the Principality of Turov-Pinsk, the Principality of Kiev, the Golden Horde, the Kingdom of Hungary, the Kingdom of Poland, the Principality of Moldova, and the Monastic State of the Teutonic Knights.