Ranger Smith wears one. Smokey Bear wears one. I wear one, too.
The Stetson flat hat is an iconic part of the park ranger uniform. I currently have the spring/summer version of the hat, which is made of straw. The fall/winter version of the hat is made of felt. Both hats are decorated by a leather hatband decorated with sequoia pine cones and branches, along with the initials USNPS. The felt hat is the original and has been in use since 1911, while the basic design of the hatband dates from 1930. The straw hat is relatively newer, dating from 1959.
You can learn more about the history of the NPS hat and NPS hatband on the National Park Service website.