A promise ring is most commonly worn on the ring finger of the left hand, which is the fourth finger, as it symbolizes love, commitment, and a shared future together. However, it can also be worn on the ring finger of the right hand, especially if the wearer is already married or engaged, or simply based on personal preference. There are no strict rules, and some people choose to wear a promise ring on other fingers like the pinkie or middle finger for personal or symbolic reasons.