The purpose of this study was to develop and test psychometrically a self-report instrument designed to measure sense of belonging in adults. The Sense of Belonging Instrument (SOBI) is a 27-item, self-report instrument consisting of two separately scored scales, SOBI-P (psychological state) and SOBI-A (antecedents). Content validity was assessed by a panel of experts. Construct validity, internal consistency, and retest reliability were examined through a series of studies with three subject groups: community college students, patients in treatment for major depression, and Roman Catholic nuns. Results suggest that SOBI-P is a valid and reliable measure of sense of belonging. SOBI-A appears to reflect an individual's motivation for sense of belonging but requires additional study regarding its construct validity and internal consistency.