Excellent medication adherence (greater than 95%) is required for optimal HIV treatment success. This study aimed to develop and validate a brief scale to assess psychological readiness for successfully starting and adhering to HIV medications. HIV-positive men and women (N = 142) from an HIV outpatient clinic completed the proposed HIV Medication Readiness Scale (HMRS) prior to starting HIV medications. The 10-item HMRS demonstrated high internal consistency (alpha = 0.90), test-retest reliability (r = 0.83), and sensitivity to change following a standardized 4-session psychoeducational intervention designed to increase readiness for successful adherence. Predictive validity was supported by higher readiness scores on the day starting HIV medications, predicting higher treatment adherence at 1-month follow-up. The HMRS is a brief, easy-to-use, clinically relevant tool that can assist in identifying people living with HIV at high risk of nonadherence, who might benefit from tailored readiness counseling prior to initiating HIV medications. (Contains 2 tables.) [This study was supported by funds from the AIDS Bureau of Ontario and Canadian Institutes of Health Research (CIHR).]