<p>Evidence supporting the effectiveness of non-pharmacological exercise interventions to mitigate the side effects of aromatase inhibitor (AI) therapy among breast cancer (BC) patients remains limited. Given that AI-related adverse effects are associated with reduced quality of life, there is a clear need to address these symptoms. Thus, this study investigated the effects of the Pilates Method (PM) intervention on thoracic spine extensor myofascial stiffness (M-FS), pain sensation, and quality of life (QoL). A randomized, two-arm, parallel-group supportive care trial with blinded outcome assessment compared an exercise intervention group (experimental group;&#xa0;EG) and a usual-care group (U-CG) pre- and post-intervention (per-protocol analysis). Eligible BC patients receiving AI therapy were randomized to either group. The EG completed a 12-week Pilates mat intervention (two supervised group sessions per week, 60&#xa0;min each). Primary outcomes included measurement M-FS of thoracic spine extensor muscles (MyotonPro®) at Th3, Th7, and Th12 spine levels, alongside assessments of pain sensation (The Brief Pain Inventory), and QoL (EORTC QLQ-C30, EORTC QLQ-BR23). Between-group and within-group differences were analyzed using a two-way repeated-measures ANOVA, followed by Bonferroni post hoc tests. 42 BC women were randomized to the EG (n = 28) and the U-CG (n = 14); 22 and 14 participants, respectively, were included in the final analysis. Significant time × group interaction effects for M-FS were observed at the Th3 (F = 17.04, η_p² = 0.33, p &lt; 0.001), Th7 (F = 20.13, η_p² = 0.37, p &lt; 0.001), whereas the interaction at Th12 was not statistically significant (F = 1.15, η_p² = 0.03, p &gt; 0.01). Significant time × group interaction effects with large effect sizes were found for pain severity, physical interference, and affective interference (all p &lt; 0.001), whereas pain indicators increased in the U-CG. QoL improved in the EG, reflected by higher global health status (GHS) (F = 18.69, η_p² = 0.36, p &lt; 0.001), with significant improvements in systemic therapy side effects, arm symptoms (both p &lt; 0.001). Clinically meaningful changes were observed in GHS and BC-specific symptom scales in the EG. No serious adverse events were observed. A supervised PM-based intervention may improve selected treatment-related musculoskeletal outcomes in BC women receiving AI. The observed reductions in thoracic extensor M-FS (Th3, Th7), accompanied by enhancements in quality of life, substantiate the potential clinical significance of Pilates-based physiotherapy as an adjunctive rehabilitation approach within this population. Given its observed analgesic effects, the PM may represent a complementary supportive strategy alongside guideline-recommended aerobic and resistance training interventions in oncology rehabilitation.</p><p><b>Trial registration:</b> Clinicaltrials.gov (No: NCT06419023; <a href="https://clinicaltrials.gov/study/NCT06419023">https://clinicaltrials.gov/study/NCT06419023</a>). May 20, 2024. Retrospectively registered.</p>