This study examined the role of friendship sex composition, friendship history, and threat appraisals in the experience of jealousy about a romantic partner’s involvement in extradyadic friendships. Using a survey, 201 individuals responded to scenarios describing a romantic partner’s involvement in a significant friendship outside the romantic dyad. A partner’s involvement in a cross-sex friendship was associated with greater perceptions of threat to both the existence and quality of the romantic relationship than was a partner’s involvement in a same-sex friendship. Further, the specific forms of jealousy experienced about partners’ friendships were dependent on the threat appraisals individuals associated with the friendships. Appraisals of relational existence threat mediated the influence of friendship characteristics (i.e., sex composition and history) on sexual jealousy and companionship jealousy, while appraisals of relational quality threat mediated the influence of friendship characteristics on intimacy jealousy, power jealousy, and companionship jealousy. This study points toward the central role of threat appraisals in mediating associations between rival characteristics and various forms of jealousy about a partner’s friendships.