An optio (plural optiones; , from optāre, "to choose", because an optio was chosen by his centurion), sometimes anglicized option (though rarely, to avoid confusion with "option"), was a soldier in the Roman army who held a position similar to that of an executive officer in modern armies. The main function of an optio was as an optio centuriae, the second-in-command of a century, although there were many other positions an optio could hold.

Optiones were vital units in the late Roman army, as casualties were often high among centurions due to their position at the front of the century, which made them easy targets. 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, supervising his subordinates, and a variety of administration duties.

Optio pay was double the standard legionary pay and they were the most likely men to be promoted after a death of a centurion.

Types of optio

Titles held by optiones included:

• Optio ad carcerem: chosen man on prison duty (incarceration).
• Optio ad spem ordinis: optio being groomed for promotion to the rank of centurion.
• Optio candidatus: optio being groomed for promotion to the rank of centurion.
• Optio carceris: soldier in charge of the prison cells.
• Optio centuriae: 'chosen man of the centuria'; second-in-command and rear rank officer of a centuria; classed as a duplicarius, a soldier receiving double basic pay; he carried a hastile (wooden staff).
• Optio centurionis: 'chosen man of the centurion'; same as optio centuriae.
• Optio custodiarum: soldier in charge of guard posts.
• Optio draconarius: 'chosen man among the dragon bearers', a late Roman senior standard bearer.
• Optio equitum: optio in the legionary or Praetorian cavalry (equus=horse).
• Optio fabricae: soldier in charge of a workshop.
• Optio navaliorum: soldier in charge of boats.
• Optio praetorii: soldier attached to headquarters.
• Optio principalis: optio who held supervisory rank (principales from the 2nd century onwards); not all who had the title of optio held this status.
• Optio speculatorum: optio in the elite cavalry bodyguards.
• Optio spei: optio being groomed for promotion to the rank of centurion.
• Optio statorum: optio of military police.
• Optio tribuni: assistant to a tribune.
• Optio valetudinarii: orderly in charge of a hospital.