Fred "Fritz" Klein (December 27, 1932 – May 24, 2006) was an Austrian-born American psychiatrist and sex researcher who studied bisexuals and their relationships.

[1] Fritz Klein founded the American Institute of Bisexuality in 1998, which is continuing his work by sponsoring bisexual-inclusive sex research, educating the general public on sexuality, and promoting bisexual culture and community.

He studied medicine at University of Bern in Switzerland for six years, receiving his MD in 1961.

Self-identifying as bisexual, Klein was surprised at the lack of literature on his sexuality in the New York Public Library in 1974.

Klein published The Bisexual Option: A Concept of One Hundred Percent Intimacy in 1978, based on his research.