The present paper introduces an index of joy, a practitioner-focused measure of behaviors associated with child assent, engagement, and positive affect during applied behavior analysis (ABA) sessions. Using a three-point scale, the index assesses practitioner support for assent, meaningful interaction, and child choice alongside child affect. Following feasibility testing and practitioner feedback, five content experts reviewed the revised index. Most items achieved 0.80 agreement; items below threshold were revised. Interobserver agreement averaged 88% (80–100%) for child affect and 85% (78–93%) for practitioner behavior. The index may support reflective practice, supervision, training, and research on compassionate, assent-based ABA.