Dionysius is a lunar impact crater that lies on the western edge of the Mare Tranquillitatis.

The rim of Dionysius is generally circular and shows little sign of wear.

The crater possesses a small ray system with a radius of over 130 kilometers.

The formation has a high albedo and appears bright when the Sun is nearly overhead during a full Moon.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Dionysius.