The Menapii are a Belgic tribe of northern Gaul in pre-Roman and Roman times.

Their territory according to Strabo, Caesar and Ptolemy stretches from the mouth of the Rhine in the north, and southwards along the west of the Schelde.

Their civitas under the Roman empire is Cassel (northern France), near Thérouanne.

(The positions of such Roman tribal capitals did not always correspond to pre-Roman political geography.)

In later geographical terms their territory corresponds roughly to the modern coast of Flanders, extended into neighboring France and possibly the Netherlands.