Quintus Marcius Rex was a member of the Marcii Reges, the family founded by the Roman King Ancus Marcius. His father as praetor in 144 BC built the Aqua Marcia aqueduct, the longest aqueduct of ancient Rome. The aqueduct was known for its water purity and its cold temperature.

Marcius carried on war against the Stoeni, a Ligurian people at the foot of the Alps, and obtained a triumph in the following year on account of his victories over them. During his consulship, Marcius lost his only son, a youth of great promise, but had such mastery over his feelings as to meet the senate on the day of his son's burial, and perform his regular official duties.

His daughter Marcia married Gaius Julius Caesar II. They had a son Sextus Julius Caesar (a politician and supporter of Gaius Marius); a daughter Julia Caesaris, wife of Gaius Marius and another son Gaius Julius Caesar III, a praetor who would marry Aurelia Cotta and would became parents to Julius Caesar the dictator. A descendant of Marcius is that consul of 56 BC, Lucius Marcius Philippus.

This article incorporates text from the public domain Dictionary of Greek and Roman Biography and Mythology by William Smith (1870).