Friendships make us happy, keep us healthy, and can even facilitate our reproductive fitness. But most friendships are not forever—even when we want them to be. How do people maintain valued friendships? I propose that “friendship jealousy” arises when people perceive others as posing threats to valued friendships, and that this response can function to prevent friendship loss and friend defection. In preliminary experiments, I tested predictions derived from this functional view. As predicted, I found, first, that friendship jealousy is calibrated to friend value. Second, friendship jealousy predicts intentions to “friend guard” (i.e., engage in behavior to protect the friendship). Third, friendship jealousy has sex-differentiated features, which are consistent with sex differences in friendship structures and ancestral friendship functions. The present work pits against one another intuitive and functional predictions as to what drives friendship jealousy. Although intuition might lead one to expect greater jealousy when a friend spends more time with a new person, a functional view suggests greater jealousy when that new person threatens to fulfill the same function for one’s friend that one is currently fulfilling (i.e., to “replace” him/her). Preliminary studies revealed that greater friendship jealousy is evoked when friends form new same-sex friendships (which presumably pose greater replacement threat, but lesser time threat) versus new romantic relationships (which presumably pose lesser replacement threat, but greater time threat). The focal experiment explicitly and experimentally manipulates a version of “replacement threat” (whether the best friend “chooses” the new friend over you) and “time threat” (how much time the best friend spends with the new friend). In line with functional predictions, the amount of time the best friend spends with a new friend drives friendship jealousy—but only when direct information about replacement threat is unavailable. Regardless of the time threat posed, participants report high friendship jealousy when replacement threat is high, and low friendship jealousy when replacement threat is low. Results imply that friendship jealousy is calibrated to replacement threat (over time threat). Overall, findings suggest that friendship jealousy might be a functional response aimed at facilitating friendship maintenance.