The Transitional Pain Service (TPS) at Toronto General Hospital is a multidisciplinary treatment program that aims to prevent chronic post-surgical pain (CPSP) and persistent opioid use after surgery. CPSP is a major public health concern impacting from 10% up to 70% of patients, depending on the type of surgery. Those experiencing CPSP are at higher risk for prolonged opioid use, which introduces challenges like addiction, misuse, and overdose. Acceptance and Commitment Therapy (ACT) is an evidence-based psychology intervention that is effective in reducing patients' risk of CPSP and opioid use. However, access to this intervention is currently limited to predominantly in-person, specialized hospital-based clinic settings like the TPS, which prioritize patients at highest risk for CPSP and often require physician referrals for quick access. There is a need for such treatment approaches to spread to other institutions and to be available for lower-risk post-surgery patients, yet the shortage of specialized pain psychologists creates a barrier to widespread dissemination. The current project will evaluate the feasibility of a novel fully self-guided online ACT intervention entitled Advancing Online Psychology Tools for the Transitional Pain Service (ADOPT-TPS), developed on a mobile health application platform called Manage My Pain (MMP). A randomized, controlled pilot feasibility trial will evaluate the efficacy of ADOPT-TPS by comparing it to a pre-existing psychologist-guided workshop. Once tested, this scalable, evidence-based online intervention can be easily implemented at institutions across Canada and beyond to address CPSP and opioid use without the need for specialized pain psychologists on staff.