Research on the connection between relationship quality and body weight yields mixed results, suggesting the presence of a moderating variable. We tested whether the connection between four measures of relationship quality (conflict, ambivalence, maintenance, and love) and body mass index (BMI) was moderated by gender and couples’ unhealthy behaviors. The results showed that among women, those reporting good relationship quality had lower BMI than those with poor relationship quality, but only if they also showed little evidence of couples’ unhealthy behaviors. In other words, only the combination of high relationship quality and low prevalence of couples’ unhealthy behaviors was linked to lower BMI among women. In contrast, low relationship quality, regardless of couples’ unhealthy behaviors, and the combination of high relationship quality and high prevalence of couples’ unhealthy behaviors were all linked to higher BMI among women. These associations were not observed among men. The results suggest the presence of symptom-system fit—a relational pattern linking health detrimental behaviors and higher relationship quality.