Objective: The purpose of this study was to examine whether attention-deficit/hyperactivity disorder (ADHD) symptom levels in college undergraduates are associated with poorer romantic relationship quality, and to test whether emotion regulation difficulties, perceived stress, and hostile relationship conflict mediate this association. Participants: The sample consisted of 189 undergraduate students aged 18 to 25. Methods: Self-report measures of ADHD symptoms, relationship quality, and the proposed mediators were collected via online survey from May through August 2011. Results: Participants who reported clinically significant levels of both hyperactivity-impulsivity and inattentiveness (consistent with ADHD-C) had lower relationship quality than those whose self-reported symptoms indicated no ADHD diagnosis. Further, for women only, both hyperactive-impulsive and inattentive symptom levels were negatively associated with relationship quality. Emotion regulation problems and hostile relationship conflict mediated this association. Conclusion: Findings suggest that ADHD impairs relationship quality among young adults and suggest mechanisms through which this impairment might occur.