Abstract Yoga has been shown to have consistent but modest effects in reducing pain and improving function in populations with chronic low back pain (CLBP) in multiple randomized controlled trials. Reviews and practice guidelines support yoga as an evidence-based treatment for CLBP with at least moderate benefit and yoga is being increasingly applied as an integrative therapy. However, the mechanisms through which yoga exerts clinical improvements on pain severity and interference have not been identified; such identification might lead to optimizing yoga interventions to improve their potency. To identify underlying mechanisms associated with yoga interventions (PA 18-323) and based on a comprehensive theoretical emotion regulation model of yoga developed by the PI, we aim to test emotion regulation as a primary mechanism of yoga's effects, and to test biological pathways through which yoga's effects on increased adaptive emotion regulation may operate to affect pain-related functioning. Emotions strongly influence perceptions of pain intensity and predict disability, particularly among individuals with CLBP, and interventions that promote emotion regulation skills have been shown to reduce pain. Increasing evidence demonstrates that consistent yoga practice can promote improved emotion regulation, but research has not yet tested whether the effects of yoga practice on CLBP are due to improvements in emotion regulation. To examine this issue, we will conduct a randomized controlled trial of 204 people with CLBP assessed pre-intervention, 6 weeks, post-intervention and at 3- and 6-month follow-up to test the impact of a yoga intervention on emotion regulation relative to stretching/strengthening without meditation or breathwork. Secondarily, we will also assess whether yoga's effects on CLBP (improved pain severity and functioning) are mediated by changes in emotion regulation. We also examine whether the link between emotion regulation and pain severity and functioning is moderated or mediated by pain sensitization, effects that have been theorized and/or demonstrated to account for changes in pain, thereby testing emotion regulation as a key mechanism underlying the clinical effects of yoga on CLBP.