BackgroundAromatase inhibitors (AI) are frequently used to treat hormone-receptor-positive breast cancer, but they have multiple adverse effects (e.g., osteoporosis, arthralgia), resulting in premature therapy discontinuation/switch. Physical activity (PA) can attenuate these negative effects and improve quality of life (QoL). However, most cancer survivors fail to perform/sustain adequate PA levels, especially in the long-term. Theory-based interventions, using evidence-based behavior change techniques, aimed at promoting long-term behavior change in breast cancer survivors are effective, but remain scarce and fail to promote self-regulatory skills and better-quality motivations associated with sustained PA adoption. This paper describes the design of the PAC-WOMAN trial, which will test the long-term effectiveness and cost-effectiveness of two state of the art, group-based interventions encouraging sustained changes in PA, sedentary behavior, and QoL. Additional aims include examining the impact of both interventions on secondary outcomes (e.g., body composition, physical function), and key moderators/mediators of short and long-term changes in primary outcomes.MethodsA 3-arm pragmatic randomized controlled trial, involving a 4-month intervention and a 12-month follow-up, will be implemented, in a real exercise setting, to compare: 1) brief PA counseling/motivational intervention; 2) structured exercise program vs. waiting-list control group. Study recruitment goal is 122 hormone-receptor-positive breast cancer survivors (stage I-III), on AI therapy (post-primary treatment completion) ≥ 1 month, ECOG 0–1. Outcome measures will be obtained at baseline, 4 months (i.e., post-intervention), 10 and 16 months. Process evaluation, analyzing implementation determinants, will also be conducted.DiscussionPAC-WOMAN is expected to have a relevant impact on participants PA and QoL and provide insights for the improvement of interventions designed to promote sustained adherence to active lifestyle behaviors, facilitating its translation to community settings.Trial registrationApril 20, 2023 – NCT05860621.April 21, 2023 –https://doi.org/10.17605/OSF.IO/ZAQ9NApril 27, 2023 – UMIN000050945.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12885-023-11137-1.