Evolutionary psychological research has studied romantic jealousy extensively within monogamous relationships, but has largely ignored jealousy among partners who mutually consent to forming extra-pair relationships (i.e., consensual non-monogamy; CNM). We examined monogamous (n = 529) and CNM (n = 159) individuals' reactions to imagining their romantic partner(s)'s extra-pair involvement. For each romantic partner, men and women completed measures of relationship jealousy and reacted to scenarios of their partner's extra-pair emotional and sexual involvement. Scenarios prompted participants to indicate which type of involvement would be more distressing and more enjoyable. They also described whether or not participants had consented to their partner's extradyadic relationship. Monogamous men were more distressed by a partner's extradyadic sexual versus emotional involvement (and a partner's emotional involvement was more enjoyable) whether the scenario was consensual or not. Monogamous women were more distressed by a partner's emotional versus sexual involvement (and a partner's sexual involvement was more enjoyable) for consensual, but not non-consensual, scenarios. There were no gender differences among CNM participants. Monogamous individuals reported greater emotional distress toward a partner's imagined extradyadic involvement, whereas CNM individuals reported thinking about their partner's extra-pair relationships more frequently. Monogamous (vs. CNM) individuals reported greater confidence that their partner would never cheat on them (i.e., enter another relationship without their consent), and CNM participants were more confident that their primary versus secondary partner would never cheat, although this effect was stronger among CNM women. Moreover, CNM participants rated that it was more important that their primary versus secondary partner did not cheat, and reported greater distress imagining that their primary versus secondary partner had cheated. Women in CNM relationships rated it more important that their partner did not cheat sexually than emotionally. Finally, we replicated previous research showing that monogamous individuals mate guard more than CNM individuals, who mate guard their primary versus secondary partner more frequently. Future directions for developing evolutionary and romantic relationship research on CNM are discussed.