This is a list of mammals for which there is documented evidence of homosexual behavior. These animals have been observed practicing homosexual courtship, sexual behavior, affection, pair bonding, or parenting.

Bruce Bagemihl writes that the presence of same-sex sexual behavior was not officially observed on a large scale until the 1990s due to possible observer bias caused by social attitudes towards LGBT people, which made homosexuality in animals a taboo subject.^[3][4] He devotes three chapters, "Two Hundred Years at Looking at Homosexual Wildlife", "Explaining (Away) Animal Homosexuality", and "Not For Breeding Only" in his 1999 book Biological Exuberance to the "documentation of systematic prejudices" where he notes "the present ignorance of biology lies precisely in its single-minded attempt to find reproductive (or other) "explanations" for homosexuality, transgender, and non-procreative and alternative heterosexualities.^[5] Petter Bøckman, academic adviser for the Against Nature? exhibit, stated "[M]any researchers have described homosexuality as something altogether different from sex. They must realise that animals can have sex with who they will, when they will and without consideration to a researcher's ethical principles". Homosexual behavior is found amongst social birds and mammals, particularly the sea mammals and the primates.^[4]

Animal sexual behavior takes many different forms, even within the same species and the motivations for and implications of their behaviors have yet to be fully understood. Bagemihl's research shows that homosexual behavior, not necessarily sexual activity, has been documented in about 500 species as of 1999, ranging from primates to gut worms.^[3][6] Homosexuality in animals is controversial with some social conservatives because it asserts the naturalness of homosexuality in humans, while others counter that it has no implications and is nonsensical to equate animal behavior to morality.^[7][8] Animal preference and motivation is inferred from behavior, thus homosexual behavior has been given a number of terms over the years. Modern research^{[9][10][11][12]} applies the term homosexuality to all sexual behavior (copulation, genital stimulation, mating games and sexual display behavior) between animals of the same sex.

This is a list of some mammals that have been recorded engaging in homosexual behavior, which is part of a larger list of animals displaying homosexual behavior including birds, insects, fish, etc.

• Male bonnet macaques, similar to the youthful ones pictured, "give each other hand-jobs and sometimes eat the resulting semen"^[1] although using "hand-job" can be seen as overly anthropomorphic.^[13]
• Ugandan kob - "Female kob perform oral sex on each other and even stroke each other's vulvas with their forelegs. They may exhibit urolagnia during sex, one female will urinate while the other sticks her nose in the stream."^[1][14]
• "Elevated levels of testosterone in utero"^[15] increases aggressiveness and both male and female spotted hyenas mount submissive same-sex members who likely have lower levels of testosterone from their mothers.^[16][17][18]
• In "slip-and-slide" orgies, groups of male grey whales, roll in the ocean rubbing their bellies against each other so that their genitals are touching.^[19][20]

• Animal sexual behavior § Mammals
• Homosexual behavior in animals § Mammals
• List of birds displaying homosexual behavior