Recent work has identified people’s social identification with their groups as a powerful positive predictor of psychological well-being. Expanding upon this, we examine the relationship between people’s social identification, their perceptions of their own relative levels of in-group prototypicality with their group (i.e., how “in-group like” they see themselves) and their psychological well-being. Two studies measured university students’ social identification and perceived self-in-group prototypicality as either a resident of their university dormitory or as a student of their university. Students then completed several measures of psychological well-being (e.g., satisfaction with life, self-esteem, positive affect; depression, anxiety, stress). Results across both studies demonstrate that perceived self-in-group prototypicality and social identification combine multiplicatively, so that people who have both high social identification and high perceptions of self-in-group prototypicality also have the highest level of psychological well-being. This work affirms the relative importance of people’s group memberships in psychological well-being. We discuss the findings with reference to people’s attachment to their groups and their perceived positions within those groups as important contributors to psychological well-being. We also consider the importance of the psychological valence of these groups and how this may impact upon the pattern of results. (PsycInfo Database Record (c) 2020 APA, all rights reserved)