Self-regulation is instantiated by theta oscillations (4 to 8 Hz) in neurons of frontal midline brain regions. Frontal midline theta (FMΘ) is inversely associated with default mode network (DMN) activation, which subserves self-referential processing. Addiction involves impaired self-regulation and DMN dysfunction. Mindfulness is an efficacious self-regulatory practice for treating addiction, but little is known about the mechanisms by which mindfulness reduces addictive behavior. In this mechanistic study of long-term opioid users (N = 165), we assessed meditation-induced FMΘ as a mediator of changes in opioid misuse. Relative to a supportive psychotherapy control, participants treated with Mindfulness-Oriented Recovery Enhancement (MORE) exhibited increased FMΘ during a laboratory-based meditation session. FMΘ during meditation was associated with self-transcendent experiences characterized by ego dissolution, nondual awareness, and bliss. MORE's effects on decreasing opioid misuse were mediated by increased FMΘ. Given the role of aberrant self-referential processing in addiction, mindfulness-induced endogenous theta stimulation might "reset" DMN dysfunction to inhibit addictive behavior.