Aromatase inhibitors (AIs) are standard post-treatment care for postmenopausal breast cancer survivors (BCS) with hormone-receptor positive (HR+) tumors. AIs improve survival and reduce risk for cancer recurrence, but they also cause significant adverse effects. The most common is arthralgia—joint pain and stiffness that affects about 50% of BCS beginning soon after starting an AI. Painful arthralgia causes emotional distress and poor health-related quality of life (HRQoL). It is also the AI side effect most strongly linked to AI adherence problems, which are a critical medical concern. Current medical and behavioral interventions for managing painful arthralgia help only a minority of BCS, cause toxicities that limit their use, or have weak evidence, revealing a critical gap limiting ability to reduce the impact of painful arthralgia on BCS using AIs. We propose to fill this critical gap with Pain Coping Skills Training (PCST), a cognitive-behavioral therapy (CBT)-informed intervention. To our knowledge, no study has evaluated PCST for management of AI-associated painful arthralgia, although randomized controlled trials (RCTs) show that PCST and similar interventions delivered by trained therapists can improve pain and reduce disability in people with cancer, chronic musculoskeletal pain, and other pain conditions. Despite efficacy, PCST continues to be underused in clinical care due to barriers such as high resource costs, lack of availability in some geographic areas, travel requirements for patients, and a shortage of trained therapists. We developed a web-based PCST program called painTRAINER to overcome these barriers, using a novel approach to optimize engagement and retain therapeutically critical features of therapist-delivered PCST in an “automated” program that does not require therapist involvement. PainTRAINER mimics highly interactive and personalized therapist-delivered PCST. RCT evidence shows that painTRAINER is highly acceptable and engaging, and that it can reduce joint pain and stiffness in people with osteoarthritis (OA). We now seek to evaluate whether it can improve the severity of painful arthralgia and its associated interference and other adverse impacts on BCS on AI therapy. The proposed multi-site RCT will randomize 452 BCS with AI- associated painful arthralgia to enhanced usual care (EUC) or painTRAINER+EUC. Assessments will occur at baseline, post-intervention (12 weeks after baseline), and 3- and 6-months post-intervention. Aims are to determine whether painTRAINER+EUC, compared to EUC, improves pain severity and pain interference (Aim 1, primary outcomes) as well as distress, HRQoL, and AI adherence (Aim 2, secondary outcomes). We will also determine whether these effects on primary and secondary outcomes are at least partially mediated by increases in pain self-efficacy and reductions in pain catastrophizing (Aim 3). Finally, for an exploratory aim we will evaluate beneficial indirect effects on sleep problems an...