Background: Usability—the extent to which an intervention can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction—may be a key determinant of implementation success. However, few instruments have been developed to measure the design quality of complex health interventions (i.e., those with several interacting components). This study evaluated the structural validity of the Intervention Usability Scale (IUS), an adapted version of the well-established System Usability Scale (SUS) for digital technologies, to measure the usability of a leading complex psychosocial intervention, Motivational Interviewing (MI), for behavioral health service delivery in primary care. Prior SUS studies have found both one- and two-factor solutions, both of which were examined in this study of the IUS. Method: A survey administered to 136 medical professionals from 11 primary-care sites collected demographic information and IUS ratings for MI, the evidence-based psychosocial intervention that primary-care providers reported using most often for behavioral health service delivery. Factor analyses replicated procedures used in prior research on the SUS. Results: Analyses indicated that a two-factor solution (with “usable” and “learnable” subscales) best fit the data, accounting for 54.1% of the variance. Inter-item reliabilities for the total score, usable subscale, and learnable subscale were α = .83, α = .84, and α = .67, respectively. Conclusion: This study provides evidence for a two-factor IUS structure consistent with some prior research, as well as acceptable reliability. Implications for implementation research evaluating the usability of complex health interventions are discussed, including the potential for future comparisons across multiple interventions and provider types, as well as the use of the IUS to evaluate the relationship between usability and implementation outcomes such as feasibility. Plain language abstract: The ease with which evidence-based psychosocial interventions (EBPIs) can be readily adopted and used by service providers is a key predictor of implementation success, but very little implementation research has attended to intervention usability. No quantitative instruments exist to evaluate the usability of complex health interventions, such as the EBPIs that are commonly used to integrate mental and behavioral health services into primary care. This article describes the evaluation of the first quantitative instrument for assessing the usability of complex health interventions and found that its factor structure replicated some research with the original version of the instrument, a scale developed to assess the usability of digital systems.