Social identities play an important role in many aspects of life, not least in those pertaining to health and well-being. Decades of research shows that these relationships are driven by a range of social identity processes, including identification with groups, social support received from groups, and multiple group memberships. However, to date, researchers have not had access to methods that simultaneously capture these social identity processes. To fill this void, this article introduces an online Social Identity Mapping (oSIM) tool designed to assess the multidimensional and connected nature of social identities. Four studies (total N = 721) featuring community, student, new parent, and retiree samples, test the reliability and validity of oSIM. Results indicate that the tool is easy to use, engaging, has good internal consistency as well as convergent and discriminant validity, and predicts relevant outcomes across a range of contexts. Furthermore, using meta-analytic findings, the tool is able to index a higher-order social identity construct, here introduced as a supergroup. This new concept provides holistic information about groups (reflecting an integrated index of several social identity processes) that are predictive of well-being outcomes, as well as outcomes related to successful adjustment to challenging life events. We discuss how the tool can be used to tackle key debates in the literature and contribute to theory by affording researchers the opportunity to capture the nuanced and contextual nature of social identity in action. (PsycINFO Database Record (c) 2020 APA, all rights reserved).