People in romantic relationships tend to have positive feelings toward their partner and want their relationship to last. However, maintaining a romantic relationship over time is challenging, and people can often experience mixed and conflicting feelings (i.e., ambivalence) toward their significant other. While research has identified the serious consequences that ambivalence can have for personal and relational well-being, very little is known about the factors that can lead people to experience ambivalence in relationships. The present work examines how extradyadic desire (i.e., desire for someone other than the partner), a common difficulty people face in the context of monogamy, is a situation in which people feel more ambivalent toward their partner. In three studies (N = 1,178) using experimental, daily diary, and longitudinal approaches, we find that feelings of desire for an attractive alternative increase ambivalence toward the current partner, above and beyond how much people actually value their partner, and that this has short- and long-term negative consequences for personal and relational well-being. Furthermore, while most people could identify an attractive alternative in their life, desire for the alternative-rather than just their presence-seems to play a stronger role in increasing ambivalence. This work highlights the emotional processes through which attractive alternatives pose a threat to romantic relationships and the role that ambivalence plays in daily life and over time. (PsycInfo Database Record (c) 2022 APA, all rights reserved).