Djelfa, town, north-central Algeria, in the Oulad Naïl Mountains at an elevation of 3,734 feet (1,138 metres). It is situated between the towns of Bou Saâda and Laghouat. Djelfa town is at a point of transition between the dry, steppelike High Plateaus of the north, with their chotts (intermittent salt lakes), and the Sahara (south). The town was founded in 1852 as a French military post on a geometric plan. It serves as an important livestock market centre for the seminomadic Arab Ouled Naïl confederation.
The surrounding region for centuries has been the meeting place of the Ouled Naïl, who live in black-and-red striped tents and claim descent from the Prophet Muḥammad. The area is notable for its abundance of Neolithic rock carvings dating from 7000 to 5000 bce. North of Djelfa town there is an imposing physical feature known as Salt Rock (Rocher de Sel) that resulted from the erosion of rock salts and marls by rain, and to the west of the town Megalithic funerary structures are found. Pop. (1998) 154,265; (2008) 265,833.