Unicorn hunting is a term used to describe the practice of a couple, usually a heterosexual couple, seeking a third partner for the relationship, either temporarily or permanently, usually a bisexual woman, either to join a threesome or to start a polyamorous relationship. The term "unicorn" comes from how rare a bisexual woman willing to join an existing couple is, as well as their conditions, which can include relational exclusivity, equal attraction to both partners, or interest in only having group sex. Unicorn hunting is generally frowned upon within the polyamorous community because it can be rooted in transphobia and objectification of the third person.

Unicorn hunting usually involves an established couple, a heterosexual man, and a bisexual woman, searching for a bisexual woman who is open to a relationship with both the man and the woman in the existing relationship, who will love them both equally, and agree to the rules that the couple has already decided are healthy for their relationship. One of the biggest problems with unicorn hunters is that they often tell the third person that they need to divide their time, affection, and sexual interaction equally between the existing partners, without offering the same to that person. This creates a catch-22 when the couple is aware theres a lack but afraid to allow anyone to genuinely fulfill it because they would then be seen as superior/replacing the existing partners place.

It is generally agreed that it is possible to practice Unicorn Hunting ethically if the fantasy being sought is purely sexual. This is because that is a very discreet activity which the "unicorn" can consent to. The hunters can treat the unicorn ethically before and afterwards. However, unicorn hunting can be problematic when it involves objectifying the third person and reducing them to gender, sexuality, and availability, and not considering their own wants and needs.