The Karasuk begins some 100 kilometres (62 mi) southwest of Novosibirsk, at 190 metres (620 ft) above sea level. It flows in a southwesterly direction through a wide valley in the southern part of the Baraba Steppe, and terminates in an endorheic basin of small lakes and swamps at an elevation of 105 metres (344 ft), at the border with Kazakhstan. At high water levels some water will flow over Chuman (Russian: Чуман) to the Burla River (Russian: Бурла), which branches from the main river towards the south, east of the town of Karasuk. Yet more water will branches off towards the north, via the Baganyonok River branch (Russian: Баганёнок), to the Bagan River. The Karasuk has no major tributaries.
At the river's lower reaches lies the town and railway hub of Karasuk, named after the river.
The source region of the Karasuk River was 26 March 2007 declared a special protected area.