Nursing home (NH) quality improvement (QI) is challenging. The critical role of NH leaders in successful QI is well established; however, current options for assessing the QI capabilities of leaders such as the licensed NH administrator are limited. This article presents the development and preliminary validation of an instrument to measure NH administrator self-efficacy in QI. We used a mixed-methods cross-sectional design to develop and test the measure. For item generation, 39 NH leaders participated in qualitative interviews. Item reduction and content validity were established with a sample of eight subject matter experts. A random sample of 211 administrators from NHs with the lowest and highest Centers for Medicare and Medicaid Services Five-Star Quality ratings completed the measure. We conducted exploratory and confirmatory factor analyses and tested the measure for internal reliability and convergent, discriminant, and known group validity. The final measure included five subscales and 32 items. Confirmatory factor analysis reaffirmed the factorial structure with good fit indices. The new measure's subscales correlated with valid measures of self-efficacy and locus of control, supporting the measure's convergent and discriminant validity. Significant differences in most of the subscales were found between the objective (Centers for Medicare and Medicaid Services Five-Star Quality rating) and subjective (Self-Rated Facility QI Index) quality outcomes, supporting the measure's known group validity. The instrument has usefulness to both NH organizations and individual NH administrators as a diagnostic tool to identify administrators with higher/lower chances of successfully implementing QI. Organizations and individuals can use this diagnostic to identify the administrator's professional development needs for QI, in general, and specific to the instrument's five subscales, informing directions for in-house training, mentoring, and outside professional development. Attending to NH administrators' QI professional development needs prior to implementing QI holds promise to enhance the chances for successful implementation of QI, which is urgently needed in many NHs.