The current study tested the predictions that (a) people project their own attraction to alternative romantic or sexual partners onto their romantic partners and (b) this projection shapes anger and negative behavior toward romantic partners. These predictions were supported in a dyadic daily experiences study of 96 heterosexual romantic couples. Participants’ self-reported attraction to alternative partners predicted perceptions of the partner’s interest independently of, and more strongly than, the partner’s own self-reported attraction, suggesting that participants projected their own extradyadic attraction onto their partners. Furthermore, this projection predicted perceivers’ own anger and negative behaviors directed at their partners more strongly than did the partner’s self-reported attraction. Results suggest that participants were angry and antagonistic when they thought their partners were interested in alternative partners, but that this suspicion was a projection of their own attraction to alternatives more than it was an accurate reflection of their partner’s attraction. Results suggest that projection of extradyadic attraction has an important influence on relationship quality and may exacerbate the negative relationship consequences of attraction to alternative partners.