Marcomer (Marcomeres, Marchomer, Marchomir) was a Frankish leader (dux) in the late 4th century who invaded the Roman Empire in the year 388, when the usurper and leader of the whole of Roman Gaul, Magnus Maximus was surrounded in Aquileia by Theodosius I.
The invasion is documented by Gregory of Tours who cited the now lost work of Sulpicius Alexander. According to this account Marcomer, Sunno and Genobaud invaded the Roman provinces Germania and Belgia in Gaul. They broke through the lines, killed many people, destroyed the most fruitful lands and made the city of Cologne panic. After this raid, the main body of the Franks moved back over the Rhine with their booty. Some of the Franks remained in the Belgian woods. When the Roman generals Magnus Maximus, Nanninus and Quintinus heard the news in Trier, they attacked those remaining Frankish forces and killed many of them. After this engagement, Quintinus crossed the Rhine to punish the Franks in their own country, however his army was surrounded and beaten. Some Roman soldiers drowned in the marshes, others were killed by Franks; few made it back to their Empire.
Nanninus and Quintinus were replaced by Charietto and Syrus, who were again confronted by an attack of unidentified Franks.
Later, after the fall of Magnus Maximus, Marcomer and Sunno held a short meeting about the recent attacks with the Frank Arbogastes, who was a general (magister militum) in the Roman army. The Franks delivered hostages as usual, and Arbogastes returned to his winter quarters in Trier.
A couple of years later when Arbogastes had seized power and the West Roman army was nearly completely in the hands of Frankish mercenaries, he crossed the Rhine with a Roman army into Germania, because he hated his own kin. Marcomer was seen as leader with Chatti and Ampsivarii but the two did not engage.