The capital of the Democratic Republic of the Congo (DRC, the larger country once called Zaire) is Kinshasa. The capital of the Republic of the Congo (often called Congo-Brazzaville) is Brazzaville.
The two capitals face each other across the Congo River just below Malebo Pool (formerly Stanley Pool): Kinshasa sits on the south bank and Brazzaville on the north bank. According to Britannica, Brazzaville is also the former capital of French Equatorial Africa, while Kinshasa is the DRC’s largest city and one of the largest cities in sub-Saharan Africa. They are among the closest pairs of national capitals in the world, apart from Rome and Vatican City.
Sources: Britannica: Brazzaville; Britannica: Kinshasa
Image: Kinshasa and Brazzaville from the International Space Station (ISS007-E-6305), NASA, public domain, via Wikimedia Commons.