Though most mourners are resilient, a minority of the bereaved experience disabling grief accompanied by clinically significant impairment in important areas of functioning. Although impairment measures exist, they have notable limitations in the context of bereavement. Therefore, the objective of this study was to develop and evaluate the Grief Impairment Scale (GIS), which was designed to identify the impact of grief on biopsychosocial functioning. The resulting 5-item scale, which was based on 363 adults who are struggling with grief, demonstrated solid reliability, factorial validity, and convergent and divergent validity with correlations with measures of impairment, prolonged grief, and psychological distress. The GIS also measures grief-related functioning equivalently across demographic groups and satisfactorily discriminates between persons with and without impairment using an optimized cut score of ≥ 9 (79% sensitivity and 74% specificity). These results provide preliminary support for the GIS as a flexible tool for clinical research and practice.