Waalo (Wolof: Waalo) was a kingdom on the lower Senegal River in West Africa, in what is 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.

Oral histories claim that, before becoming a kingdom, the area of Waalo was ruled by a patchwork of Lamanes, a Serer title meaning the original owner of the land. Etymological evidence suggests that the area was ruled by the Jaa'ogo dynasty of Takrur. This aligns with early Arabic written sources which describe an island town known as Awlil (Waalo) near the mouth of the Senegal river, in a region called Senghana.

The exact founding date of Waalo is debated by historians, but is associated with the rule of the first king, the semi-legendary Ndiadiane Ndiaye, in 1287. Ndiaye, originally a Fula from Takrur, united the Lamanes and ruled Waalo for 16 years as an arbiter or judge rather than king before, according to some oral accounts, being driven out by his half brother Barka Bo, or Barka Mbodj. After this, Ndiaye took control of Jolof and founded the Jolof Empire. Barka Mbodj was the first ruler to use the royal title 'Brak'. Ndiaye eventually made Waalo a vassal.

Europeans first appeared off the coast of Waalo in the 15th century, and soon began trading. This caused a significant shift in economic power away from the Jolof heartland towards coastal vassals such as Waalo and Cayor. Buumi Jelen, a member of the royal family, may have established his own control over Waalo during this period, and is credited with creating a system of alkaldes who served as customs collectors for the Buurba Jolof. He later attempted to ally with the Portuguese to take power, but was killed by his erstwhile allies in a dispute.

The Jolof empire broke up in the aftermath of the battle of Danki in 1549, though the Brak continued to pay symbolic tribute to the Bourba Jolof until 1715.

In 1638, the French established the first permanent European trading settlement at the mouth of the Senegal River, moving to the site of Saint-Louis in 1659 while facing consistent military and political pressure from the Brak. The French presence would have a decisive effect on the rest of the history of Waalo.

Partly in response to the shift in trade away from Berber tribes to the French, Nasr ad-Din, a Berber Marabout, launched the Char Bouba War or the Marabout War, overthrowing the ruling aristocracy of Waalo (among other Senegal river kingdoms) in an attempt to establish an Islamic theocracy. Upon his death in 1674, however, his movement collapsed and the old hierarchies, aligned with Arab Hassan tribes north of the river and vigorously supported by the French, re-asserted themselves.

During this same period, Moroccan forces came south to the Senegal river, forcing the Brak to move the capital from Ngurbel to the south bank and permanently breaking the kingdom's control on the north side.