Cassin's kingbird is a large tyrant flycatcher native to western North America. The name of this bird commemorates the American ornithologist John Cassin.

John Cassin was an American ornithologist from Pennsylvania. He worked as curator and Vice President at the Philadelphia Academy of Natural Sciences and focused on the systemic classification of the Academy's extensive collection of birds. He was one of the founders of the Delaware County Institute of Science and published several books describing 194 new species of birds. Five species of North American birds, a cicada, and a mineral are named in his honor.