The County of Pallars Jussà or Lower Pallars was a county in the Hispanic March during the eleventh and twelfth centuries, long after the march had ceased to be effectively administered by the Kings of France. It was a division of the County of Pallars, which had been de facto, and possible de jure, independent since the late ninth century. It roughly corresponded with the modern Catalan comarca of Pallars Jussà.
In 1011, Sunyer of Pallars died and by his testament his county was divided between his sons, the eldest, Raymond III, receiving Pallars Jussà and the younger, William II, receiving Pallars Sobirà. Pallars Jussà comprised the valley of Flamicell, the left bank of the Noguera Ribagorzana, and Pobla de Segur. It had a frontier with the Moorish Conca de Tremp and Montsec and its capital was at Segur.
By the year 1000, the economic and social centre of Pallars was located in the Pobla de Segur and Tremp, in Pallars Jussà. It was wealthier than Pallars Sobirà and capable of expansion in the Reconquista against the Muslim territory to its south. Raymond III, however, ignored the opportunities of war with the Moor, rather leaving the such venues open to Arnau Mir de Tost, a baron from the County of Urgell. Raymond IV, son and successor of Raymond III, was also prevented from concentrating on the border by the attacks of his neighbours, his cousin Artau I of Pallars Sobirà, Ermengol III of Urgell, and the noble family of the Vallferrera.