The Kingdom of Waalo (Oualo) was a kingdom on the lower Senegal River in West Africa, in what are now Senegal and Mauritania. It included parts of the valley proper and areas north and south, extending to the Atlantic Ocean. To the north were Moorish Emirates; to the south was the Kingdom of Cayor; to the east was Jolof (Diolof).

Waalo had a complicated political and social system, which has a continuing influence on Wolof culture in Senegal today, especially its highly formalized and rigid caste system. The kingdom was indirectly hereditary, ruled by three matrilinial families: the Logar, the Tedyek and the Joos, all from different ethnic backgrounds. Its history includes constant struggles among these families to become "Brak" or king of Waalo, as well as wars with its neighbors.

The legendary 13th century founding of Waalo was by NDiadiane Ndiaye, a man reputed to be from the north, who lived a strange existence in the Senegal River for years until he was selected by the Waalo people as their king after performing amazing feats. Later he went on to become first ruler of a united Jolof (Diolof) Empire which lasted until the mid 16th century.

The royal capital of Waalo was first Diourbel (Guribel) on the north bank of the Senegal River (in modern Mauritania), then Ndiangué on the south bank of the river, then the capital was moved to Nder on the west shore of the Lac de Guiers. Waalo was subject to constant raids for slaves not only from the Moors but also in the internecine wars.

The Brak ruled with a kind of legislature, the Seb Ak Baor, over a complicated hierarchy of officials and dignitaries. Women had high posiitons and figure promininently in the political and military history.

Waalo had lucrative treaties with the French, who had established their base at the island of Saint-Louis (now Saint-Louis, Senegal) near the mouth of the river. Waalo was paid fees for every boatload of gum arabic or slaves that was shipped on the river, in return for its "protection" of the trade.

Eventually this protection became ineffective. Vassals of Waalo, like Beetyo (Bethio) split off. Waalo was conquered in 1855 by the French, in a campaign of Governor Louis Faidherbe. This was the first major inland conquest by the French, by which they eventually obtained all of Mauritania, Senegal and French Sudan (now Mali). In all, Waalo had 52 kings since its founding.

Waalo had its own traditional African religion. The ruling class was slow to accept Islam, which had spread in the valley; the Brak converted only in the 19th century.