The Grand Erg Occidental (Arabic: العرق الغربي الكبير, al-ʿIrq al-Gharbī al-Kabīr), (also known as the Western Sand Sea) is the second largest erg in northern Algeria after the Grand Erg Oriental. It covers an area of approximately 78,000 square kilometres (30,000 sq mi). The sand dunes in the erg are formed by the wind, and can be up to 120 metres (390 ft) high. Certain crescent-shaped dunes, known as barchans, are actually mobile; the wind can push these dunes as much as 20 to 30 m (65–100 ft) in one year.

It is a desert natural region that receives less than 50 mm (1,96 in) of rainfall per year. The mean elevation of the Grand Erg Occidental is about 500 m, on average higher than the elevation of the Grand Erg Oriental, but not as high as the neighboring Tademaït to the southwest. This desolate region is a practically uninhabited area; there are no permanent villages.

• 
  Grand Erg Occidental (centre), as seen from space
• 
  Ksar of Aghlad; a Ksar -which means castle- was a structure where a tribe used to live

• Grand Erg Oriental
• Geography of Algeria
• List of ergs

30°40′N 0°00′E / 30.667°N 0.000°E / 30.667; 0.000