Niš (Serbian: Ниш, pronounced [nîːʃ] ( listen)) is the largest city of southern Serbia and the third-largest city in Serbia (after Belgrade and Novi Sad). According to the preliminary results of the 2011 census, the city has an urban population of 182,208 inhabitants, while its municipality has a population of 257,867. The city covers an area of 597 km², including the urban core, town of Niška Banja and 68 suburbs. Niš is the administrative center of the Nišava District.

It is one of the oldest cities in the Balkans and Europe, and has from ancient times been considered a gateway between the East and the West. The Paleo-Balkan Thracians first inhabited the area in the Iron Age, and Triballians dwelled here prior to the Celtic invasion in 279 BC which established the Scordisci as masters of the region. Modern-day Niš was among the cities taken in the Roman conquest in 75 BC. The Romans built the Via Militaris in the 1st century, with Naissus being one of the key towns. Niš is also notable as the birthplace of Constantine the Great, the first Christian Roman Emperor and the founder of Constantinople, as well as two other Roman emperors, Constantius III and Justin I. It is home to one of Serbia's oldest churches dating to the 4th century AD in the suburb of Mediana.