This article describes the work of a task force, commissioned by the American Psychological Association’s Society of Clinical Psychology (SCP), to define “psychological treatment.” We discuss SCP’s rationale for needing such a definition, including the potential for non-psychological interventions to be considered under the current model of empirically supported treatments. The task force, in collaboration with the membership of SCP, proposes the following definition: Psychological treatment is an intervention consisting of specific actions between a person or persons and a mental health professional or designee, with the intent of engaging cognitive, emotional, behavioral, or interpersonal processes, in the service of modifying health or functional outcomes, and whose core assumptions about its procedures and mechanisms of change are founded in psychological science and consistent with scientific understanding. We outline our thinking around this definition and discuss alternatives that were rejected. The definition proposed here is largely consistent with the clinical practice guidelines of the American Psychological Association, though we note that at least one of those guidelines goes well beyond interventions that could reasonably be called psychological treatments. Potential uses and misuses of this definition are outlined, and we suggest additional areas for exploration and clarification.