Marcus Julius Cottius was king of the Ligurian tribes inhabiting the mountainous region now known as the Cottian Alps early in the 1st century BC. He was the son and successor of King Donnus, who had previously opposed but later made peace with Julius Caesar. Cottius initially maintained his independence in the face of Augustus' effort to subdue the various Alpine tribes, but finally submitted when Augustus named him prefect of the dozen tribes in his region. Cottius later honored his patron with a triumphal arch in his capital of Segusio.
Cottius was succeeded by his son Gaius Julius Donnus (II). His son, named Marcus Julius Cottius (II), was granted the title of king by the emperor Claudius. On his death, Nero annexed his kingdom as the province of Alpes Cottiae.
Another of the elder Cottius' sons was the Roman centurion Julius Vestalis, who retook the frontier post of Aegyssus (modern Tulcea) on the Danube after it was captured by the Getae, a deed celebrated by Ovid in his Epistulae ex Ponto IV.