An optio (, from optāre, 'to choose', so-called because an optio was chosen by a centurion; : optiones), was a position in a centuria (century) of a Roman army similar to that of an executive officer. The main function of an optio was as an optio centuriae, the second-in-command of a century, although there were many other roles an optio could adopt. Optiones were vital in the Roman army. An optio was stationed at the rear of the ranks to keep the troops in order. Their duties would include enforcing the orders of the centurion, taking over the centurion's command in battle should the need arise, supervising his subordinates, and a variety of administrative duties. Optio pay was double the standard legionary pay and they were the most likely men to replace the centurion if the position became vacant.

Types of optio

Titles held by optiones included:

Uniform

Unlike the centurion, the cuirass was not the distinguishing part of the optio's uniform. An optio's armour would be more like those of the common legionary. He could wear the lorica segmentata or a lorica hamata as well as have his gladius on the right, not the left, side. One thing that did separate him from the common legionary was the staff (called a hastile), which was used to keep the legionaries in line. This staff would be roughly as tall as the optio himself. Optiones often carried wax tablets on which they kept the orders of the day.

Vegetius on optiones

"The optiones are subaltern officers, so denominated from their being selected by the option of their superior officers, to do their duty as their substitutes or lieutenants in case of sickness or other accident."

• Flavius Vegetius Renatus