A fundamental dimension along which all social and personal relationships vary is closeness. The Unidimensional Relationship Closeness Scale (URCS) is a 12-item self-report scale measuring the closeness of social and personal relationships. The reliability and validity of the URCS were assessed with college dating couples (N = 192), female friends and strangers (N = 330), friends (N = 170), and family members (N = 155). The results show that the scale is unidimensional, with high reliability across relationship types (M α =.96). Evidence consistent with validity included substantial within-couple agreement for the romantic couples (intraclass correlation =.41), substantial friend-stranger discrimination for the female friends (η2 =.82), and measurement invariance across relationship types. Evidence of convergent and divergent validity was obtained for inclusion of other in the self and relational satisfaction, respectively. © 2011 American Psychological Association.