We examine the validity and reliability of a single-item measure of social identification (SISI). Convergent validity is shown with significant positive correlations with previously published unidimensional and multidimensional measures of in-group identification and other group-relevant measures (e.g., entitativity and collective self-esteem). Divergent validity is shown via nonsignificant correlations with social desirability measures. Predictive validity is shown with positive correlations with group-relevant behavior (e.g., volunteerism and voting). External validity is shown with correlations with other in-group identification measures in a community sample. The reliability of the scale is shown by examining scores of the SISI for six different identities at three points in time. Copyright © 2013 John Wiley & Sons, Ltd.