The Marsi (German: Marser) were a small Germanic tribe settled between the Rhine, Ruhr and Lippe rivers in northwest Germany. Tacitus mentions them repeatedly, in particular in the context of the wars of Germanicus. They had been part of the tribal coalition of the Cheruscian war leader Arminius that in 9 CE annihilated three Roman legions under Varus in the Battle of Teutoburg Forest. Germanicus, seeking revenge for this defeat, invaded the lands of the Marsi in 14 CE with 12,000 legionnaires, 26 cohorts of auxiliaries and eight cavalry squadrons. Celebrating the feast of their goddess Tanfana, the Marsi were too drunk to respond effectively to the Roman surprise attack and were massacred. According to Tacitus (Annals 1, 51), an area of 50 Roman miles was laid to waste with fire and sword: "No sex, no age found pity."

Enraged by this and other similar bloodbaths (e.g., in the spring of 15 among the Chatti), the frequently quarrelling tribes united once again to fend back the Roman invaders. After two more years of warfare, Rome finally abandoned its efforts to push its boundaries eastward to the Weser river and retreated permanently behind the Rhine.

Several town names today remain as reminders of the ancient Marsi -- e.g., Marsberg and Obermarsberg in eastern Northrhine-Westphalia and Volkmarsen in northern Hesse.

The Marsi in Germany are not to be confused with the Marsi in Italy.