The effectiveness of a competency-based supervision approach called Motivational Interviewing Assessment: Supervisory Tools for Enhancing Proficiency (MIA: STEP) was compared to supervision-as-usual (SAU) for increasing clinicians' motivational interviewing (MI) adherence and competence and client retention and primary substance abstinence in a multisite hybrid type 2 effectiveness-implementation randomized controlled trial. Participants were 66 clinicians and 450 clients within one of eleven outpatient substance abuse programs. An independent evaluation of audio recorded supervision sessions indicated that MIA: STEP and SAU were highly and comparably discriminable across sites. While clinicians in both supervision conditions improved their MI performance, clinician supervised with MIA: STEP, compared to those in SAU, showed significantly greater increases in the competency in which they used fundamental and advanced MI strategies when using MI across seven intakes through a 16-week follow-up. There were no retention or substance use differences among the clients seen by clinicians in MIA: STEP or SAU. MIA: STEP was substantially more expensive to deliver than SAU. Innovative alternatives to resource-intensive competency-based supervision approaches such as MIA: STEP are needed to promote the implementation of evidence-based practices.