The Menapii were a Belgic tribe dwelling near the North Sea, around present-day Cassel, during the Iron Age and the Roman period.
The Menapii were persistent opponents of Julius Caesar's conquest of Gaul, resisting until 54 BC. They were part of the Belgic confederacy defeated by Caesar in 57 BC, contributing 9,000 men. The following year they sided with the Veneti against Caesar. Caesar was again victorious, but the Menapii and the Morini refused to make peace and continued to fight against him. They withdrew into the forests and swamps and conducted a hit-and-run campaign. Caesar responded by cutting down the forests, seizing their cattle and burning their settlements, but this was interrupted by heavy rain and the onset of winter, and the Menapii and Morini withdrew further into the forests. In 55 BC the Menapii tried to resist a Germanic incursion across the Rhine, but were defeated. Later that year, while Caesar made his first expedition to Britain, he sent two of his legates and the majority of his army to the territories of the Menapii and Morini to keep them under control. Once again, they retired to the woods, and the Romans burned their crops and settlements. The Menapii joined the revolt led by Ambiorix in 54 BC. Caesar says that they, alone of all the tribes of Gaul, had never sent ambassadors to him to discuss terms of peace, and had ties of hospitality with Ambiorix. For that reason he decided to lead five legions against them. A renewed campaign of devastation finally forced them to submit, and Caesar placed his ally Commius of the Atrebates in control of them.
A cohort of Menapian auxiliaries is attested by inscriptions dating to the 2nd century in Britain. Carausius, the 3rd century commander of the Roman fleet who declared himself emperor of Britain and northern Gaul, was a Menapian, born in Batavia. A legion called the Menapii Seniores is mentioned in the Notitia Dignitatum, a 5th-century register of Roman government positions and military commands.
They are mentioned as Menapii by Caesar (mid-1st c.