Objective: Many treatment engagement challenges are documented for adolescents with ADHD. Across contexts, helping professionals (i.e., therapists, prescribers, educators, coaches) might benefit from an engagement strategy toolbox to facilitate work with adolescents with ADHD and their families. Method: The current study describes the development and psychometric testing of the ADHD Engagement Process Code (AEPC), a measure that operationalizes engagement strategies in the context of a blended behavioral/motivational interviewing treatment for adolescent ADHD (Supporting Teens' Autonomy Daily; STAND). The AEPC also operationalizes in-session parent and adolescent engagement-related behaviors. Behavior counts and global codes were coded for 840 audio-recorded STAND sessions delivered by 21 therapists to 121 adolescents. Subsets of tapes were double coded using the AEPC's parent, adolescent, and therapist coding systems to assess kappa for line-by-line verbalizations and intraclass correlations for session-level behavior counts and global scores. Construct validity was assessed. We explored low frequency and low variability codes and examined correlations between codes. Results: AEPC codes possessed good to excellent inter-rater reliability and strong discriminant validity. Three low frequency codes and one low variability global were identified indicating opportunities for AEPC refinement. Conclusions: The AEPC is publicly available (https://osf.io/kshfy/) and offers a library of adolescent-specific codes for those interested in measuring provider, parent, or adolescent engagement behaviors in relevant populations or contexts.