Objectives: Caregiver self-efficacy-a caregiver's belief in his/her ability to contribute to patient self-care-is associated with better patient and caregiver outcomes in single chronic conditions. It is, however, unknown if caregiver self-efficacy improves patient and caregiver outcomes in multiple chronic conditions (MCCs) because there is no instrument to measure this variable. We developed the 10-item Caregiver Self-Efficacy in Contributing to patient Self-Care (CSE-CSC) scale for that purpose, and we tested its psychometric characteristics in caregivers of patients with MCCs. Methods: In this cross-sectional multisite study, we tested the structural validity of the CSE-CSC scale with exploratory and confirmatory factor analysis, and we tested construct validity by correlating CSE-CSC scores with those of the Caregiver Contributions to Self-Care of Chronic Illness Inventory. We also tested reliability, and precision of the CSE-CSC scale. Results: The 358 enrolled caregivers (mean age 54.6 years; 71.5% female) cared for patients with an average of 3.2 chronic conditions. Structural validity was good, and it showed 2 factors within the scale. Construct validity showed significant correlations between scores of the CSE-CSC scale and the Caregiver Contributions to Self-Care of Chronic Illness Inventory. Reliability coefficients were between 0.90 and 0.97. Measurement error yielded satisfactory results. Conclusions: The CSE-CSC scale is valid, reliable, and precise in measuring caregiver self-efficacy in contributing to patient self-care in MCCs. Because caregiver self-efficacy is a modifiable variable, the CSE-CSC scale can be used in clinical practice and research to improve patient and caregiver outcomes.