Fritz Klein was a psychiatrist and an American sex researcher whose work focused on the study of bisexuality and the inclusion of bisexuality into the larger discussion of sexual orientation. He was also an activist and organizer, and he may be best known academically for the development of the Klein Sexual Orientation Grid, which sought to expand the Kinsey scale to account for a more fluid understanding of sexual orientation.
Klein founded the American Institute of Bisexuality and was the founding editor of the Journal of Bisexuality.

