Tharthar Depression is one of the largest closed depressions in Iraq; it is located in the central western part of Iraq, between the Jazira and Mesopotamia Plains, west of the Tigris River. It covers about 2050 Km2, oriented almost N – S, then changes to N35° W – S35° E, with a bowl shape, the base being in the south. The exposed rocks in the near surroundings of the depression belong to Fatha and Injana formations, with Al-Fatha Alluvial Fan sediments in its eastern bank. Tectonically, it is located in the Mesopotamia Foredeep of the Unstable Shelf, forming the contact between the Jazira and Mesopotamia Plains. Geomorphologically, it is a large depression with a floor of – 3 m, above the sea level. The maximum length and width of the depression are 120 and 48 Km, respectively. The eastern rim of the depression is higher than the western one; the heights of both rims are 90 and 75 m, respectively. The Tharthar Depression since 1956 is changed to an artificial reservoir to collect the over flooded water of the Tigris River, during flood seasons, therefore was called Tharthar Lake. The Tigris and Euphrates Rivers, however, link the reservoir, by means of artificial canals. The inlet canal, however, is from the Tigris River, regulated by Samarra Dam, which controls the divergent excess water from the Tigris River, by means of the inlet canal.The genesis of the Tharthar Depression is a matter of debit, since 1959, many ideas explain the origin of the depression. The aim of this study is to deduce the origin of the depression. This study proves that the depression is of multi genesis, closed depression formed mainly by karstification, due to dissolving of gypsum rocks of the Fatha Formation. The measured parameters, depth/ width ratio, length/ width ratio and width at top/ width at bottom ratio range from 0.017 to 0.125, 27.6 to 300, and 2 to 4.5, respectively. Such ratios assume either collapse or solution doline, of multi origin. The age of the Tharthar Depression is most probably Holocene.