An adjustable brim for use with a hat, such as a baseball-style hat includes a sheath having a rounded shape for positioning around the hat and including a channel, a body, a tightening device positioned within the channel of the sheath, and a fastener positioned on the tightening device for adjusting the size of the sheath. The body includes an inner edge, an outer edge opposite the inner edge, an outer surface, and an inner surface opposite the outer surface, and the inner edge of the body is connected to the sheath.