The Nišava or Nishava (Bulgarian and Serbian Cyrillic: Нишава, Serbian pronunciation: [nǐʃaʋa]) is a river in Bulgaria and Serbia, a right tributary, and with a length of 218 kilometres (135 mi) also the longest one, of the South Morava.

The Nišava originates in western Bulgaria, in the Stara Planina mountains (east of Kom Peak) near the village of Gintsi. Its source is close to the Serbian border. It enters Serbia after 67 kilometres (42 mi) of flow through Bulgaria without receiving any major tributaries.

Because it flows through Gintsi, the upper course of the river is known as Ginska (Cyrillic: Гинска). It first flows to the south, then sharply turns west into the Godech Kettle, passing through Razboishte, after which it forms a gorge. Coming out of the gorge, it reaches Kalotina, a major border crossing on the Bulgarian-Serbian border (Kalotina-Gradina), and continues west into Serbia.

Flowing generally to the west for the remaining 151 kilometres (94 mi), it passes near Dimitrovgrad, Pirot, Bela Palanka, Niška Banja and Niš, one of the largest cities in Serbia, 10 kilometres (6.2 mi) after which the Nišava empties into the Južna Morava. With the rapid growth of Niš in previous decades and its still fast growing suburbs, the banks of the Nišava are urbanized almost to its mouth.

The river belongs to the Black Sea drainage basin. Its own drainage area covers 4,086 square kilometres (1,578 sq mi), of which about 73% is in Serbia, the rest in Bulgaria. The Nišava is not navigable. It is not only the longest tributary of the Južna Morava, but also the largest one in terms of discharge (36 metres per second (120 ft/s)). It has many smaller tributaries, the most important being the Temštica from the right, and the Jerma (or Sukovska reka), Crvena reka, Koritnička reka and Kutinska reka from the left.

In its Serbian part, the Nišava has carved a composite valley with several depressions (Dimitrovgrad, Pirot (or Basara; Cyrillic: Басара), Bela Palanka and Niš). However, the most prominent geological feature the river has formed is the Sićevo Gorge. There are numerous caves in the step-like limestone cliffs, the majority of which remains unexplored.

The gorge is located between Bela Palanka and Niška Banja. The gorge is 17 kilometres (11 mi) long, and 350 to 400 metres (1,150 to 1,310 ft) deep. The gorge is a composite one, which means in consists of several gorges and widenings, like the Prosečka gorge, Ostrovička basin or Gradištanski canyon.

The river is quite powerful in the gorge, which is used for two power stations ("Sićevo" and "Ostrovica") used for electricity production, irrigation and fishery. The plants are old, from the first half of the 20th century. "Sićevo" was built in 1909 as "Sveta Petka", and was declared a cultural monument. In some parts canyon like-structures (like inverse valley slopes at Gradištanski canyon) were formed by the river. The gorge itself carved through the Kunovica plateau between the southern slopes of the Mountains of Svrljig and the mountain of Suva Planina, and the surrounding areas are known for their high-quality vineyards. There is also a large Ostrovica quarry in the gorge, where six villages are located, the largest one being Sićevo that gives the name to the whole gorge.