The Marsaci or Marsacii were a Germanic tribe in Roman imperial times, who lived within the area of the Rhine–Meuse–Scheldt delta, under Roman domination. (The river Meuse is the Maas in Dutch, and this name is also often used in English.)
The only relatively clear source concerning the location of this tribe is Pliny the Elder's Natural History. They are in a list of tribes living in the islands along the Rhine in its river delta region. First he mentions the islands of the Batavians and the Cananefates, and then he gives the list of people who he says are stretched out along 100 Roman miles, between Helinius and Flevus.
The Helinius (or Helinium) is understood to be a southern branch of the Rhine, connecting to the Meuse, like the modern river Waal. Flevus (or Flevum) was a Roman fortification, possibly north of the Rhine, mentioned in other sources such as Tacitus, and apparently here also refer to a branch of the Rhine, this time flowing more northerly than the main Rhine, possibly tracing a path similar to the modern Ijssel, emptying into "lakes", possibly in the location of the modern Zuiderzee.[1]
The tribes of this stretch of delta islands are mentioned in this order: Frisii, Chauci, Frisiavones, Sturii and Marsacii. Of these:-
About the Marsacii other records mention them being effected by the Batavian revolt confirming that they lived close to the Batavians. Also, like the Batavians and Cugerni, the Roman emperors recruited their horse guard from both the Frisiavones and the Marsacii.
It has been claimed on the one hand that there might be a link to an earlier named Germanic tribe, from far to the east, known as the Marsi. Somewhat more positively considered is the proposal that the name of the Marsacii is preserved in the name of a medieval gau which was named Marsum. This was to the north of the mouth of the Maas into the North Sea.