Legatus legionis was a title awarded to legion commanders in Ancient Rome.

By the time of the Roman Republic, the term legatus delegated authority (usually a consul or proconsul). Julius Caesar made wide use of the title throughout the Gallic Wars.

From Augustus, the emperor gave the title of legatus legionis to senior commanders (former Tribunes) of a legion, except in Egypt and Mesopotamia, where the legions were commanded by a praefectus legionis of an equestrian rank. The legatus legionis was under the supreme command of Legatus Augusti pro praetore of senatorial rank. If the province was defended by a single legion, the Legatus Augusti pro praetor was also in direct command of the legion.

A legatus legionis could order capital punishment.

The senatorial legatus legionis was removed from the Roman army by Gallienus, who preferred to entrust the command of a legionary unit to a leader chosen from within the equestrian order who had a long military career.

This post generally lasted 3 or 4 years, but could be much longer. A legatus legionis was usually from a wealthy or important family.