Friendships can foster happiness, health, and reproductive fitness. However, friendships end-even when we might not want them to. A primary reason for this is interference from third parties. Yet, little work has explored how people meet the challenge of maintaining friendships in the face of real or perceived threats from third parties, as when our friends inevitably make new friends or form new romantic relationships. In contrast to earlier conceptualizations from developmental research, which viewed friendship jealousy as solely maladaptive, we propose that friendship jealousy is one overlooked tool of friendship maintenance. We derive and test-via a series of 11 studies (N = 2,918) using hypothetical scenarios, recalled real-world events, and manipulation of online emotional experiences-whether friendship jealousy possesses the features of a tool well-designed to help us retain friends in the face of third-party threats. Consistent with our proposition, findings suggest that friendship jealousy is (a) uniquely evoked by third-party threats to friendships (but not the prospective loss of the friendship alone), (b) sensitive to the value of the threatened friendship, (c) strongly calibrated to cues that one is being replaced, even over more intuitive cues (e.g., the amount of time a friend and interloper spend together), and (d) ultimately motivates behavior aimed at countering third-party threats to friendship ("friend guarding"). Even as friendship jealousy may be negative to experience, it may include features designed for beneficial-and arguably prosocial-ends: to help maintain friendships. (PsycInfo Database Record (c) 2021 APA, all rights reserved).