| Official Name: | Nkayi |
| Pushpin Map: | Republic of the Congo |
| Pushpin Label Position: | top |
| Pushpin Map Caption: | Location in the Republic of the Congo |
| Subdivision Type: | Country |
| Subdivision Name: | Republic of the Congo |
| Subdivision Name1: | Bouenza Department |
| Subdivision Type2: | District |
| Subdivision Name2: | Kayes District |
| Subdivision Type3: | Commune |
| Subdivision Name3: | Nkayi |
| Area Total Km2: | 7.6 |
| Population As Of: | 2018 |
| Population Total: | 100783 |
| Population Density Km2: | auto |
| Population Blank1 Title: | Ethnicities |
| Population Blank2 Title: | Religions |
| Coordinates: | -4.1842°N 13.2883°W |
| Elevation M: | 181 |
| Area Code: | 242 |
Nkayi, also known as N'kayi, is a town and a commune[1] in the Bouenza Department of the Republic of the Congo. It is the country's fourth largest city with a population of 100,783 (2018 census).
It is divided into 2 urban boroughs (arrondissements):
Nkayi is a major sugar production centre in the country.
The city is served by a station on the national railway network, the Congo-Ocean Railway. It is home to Yokangassi Airport, also known as Nkayi Airport.
Situated in the fertile Niari Valley, the town was named Jacob in 1887. That was the name of the French engineer who led the first mission to select the best route for the Congo-Ocean Railway. Then the city began growing thanks to the thriving sugar cane plantations led by important agroindustries. In 1975, Jacob had a population of 25,000, and its name was changed to the native one of Nkayi.
The city is the fourth largest town in the Republic of the Congo since 1980.