thumb|[[Lutsk Castle, Ukraine, built by Liubartas and improved by Vytautas the Great. During Lithuanian rule the city started to prosper]]
Liubartas or Demetrius of Liubar (died ) was a Lithuanian prince from the Gediminid dynasty. He was the prince of Volhynia, and from 1320, he ruled over Lutsk, Liubar and Zhytomyr. Liubartas was also the last ruler of the Kingdom of Galicia–Volhynia.
Biography
Liubartas was the youngest son of Gediminas, Grand Duke of Lithuania. In the early 1320s he married a daughter of Andrew of Galicia and ruled Lutsk with Liubar (today town in Zhytomyr Oblast) in eastern Volhynia. At the time Boleslaw was fourteen years old and was betrothed to Eufemija, daughter of Gediminas. Liubartas continued to rule Lutsk and Volodymyr. That way the Galicia–Volhynia Wars were postponed until after Boleslaw's poisoning in 1340. Sources are too scarce to fully reconstruct events between 1341–1349.
Despite the support from his brothers Algirdas and Kęstutis, Liubartas lost all territories except for eastern Volhynia with Lutsk to Casimir III of Poland in 1349. In 1351 he was even taken prisoner during a battle, and Kęstutis had to rescue him. In 1366 a treaty was signed: Liubartas retained eastern Volhynia with Lutsk, while Poland got western Volhynia and Galicia. However the matter was settled only in 1370: Liubartas took advantage of Casimir's death and captured all of Volhynia. He supported his brother Kęstutis against nephew Jogaila during the succession fights. He built a castle in Lutsk, known as the Lubart's Castle, that survives to this day. Liubartas died , having ruled Volhynia for roughly sixty years. He married for the second time to an unnamed daughter of Konstantin of Rostov, a relative of Simeon of Moscow.