Social identity theory offers an important lens to improve understanding of founders as enterprising individuals, the venture creation process, and its outcomes. Yet, further advances are hindered by the lack of valid scales to measure founders' social identities. Drawing on social identity theory and a systematic classification of founders' social identities (Darwinians, Communitarians, and Missionaries), we develop and test a corresponding 15-item scale in the Alpine region and validate it in 13 additional countries and regions. The scale allows identifying founders' social identities and relating them to processes and outcomes in entrepreneurship. The scale is available online in 16 languages.