A baseball cap is a type of soft cap with a rounded crown and a stiff peak projecting in front.

The front of the cap typically displays a design or a logo (historically, usually only a sports team, namely a baseball team, or names of relevant companies, when used as a commercial marketing technique). The cap may be "fitted" to the wearer's head or the back may have elastic, a plastic prong-in-a-hole (multiple holes with one prong that can be inserted), Velcro, a zipper, or a tri-glide slide so that it can be quickly adjusted to fit different wearers' heads. The baseball cap is a part of the traditional baseball uniform worn by players, with the brim pointing forward to shield the eyes from the sun. Since the 1980s, varieties of the hat have become prevalent in the United States, both for utilitarian (protecting the eyes from the sun) and fashion accessory purposes.

In 1860, the Brooklyn Excelsiors wore the ancestor of the modern rounded-top baseball cap, which featured a long peak and a button on top, and by 1900, the "Brooklyn style" cap became popular.[1] During the 1940s, latex rubber became the stiffening material inside the hat and the modern baseball cap was born. The peak, also known in certain areas as the "bill" or "brim", was designed to protect a player's eyes from the sun. Typically, the peak was much shorter in the earlier days of the baseball hat. Also, the hat has become more structured, versus the overall "floppy" cap of the 19th and early 20th centuries. The baseball cap was and still is an important means by which to identify a team. Often the logo, mascot, or team's initial was placed on the cap. Usually, the cap was also fashioned in the official colors of a particular team.

The basic shape, including curved peak, is similar to some styles of 19th century sun bonnets.