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Post-stroke spasticity involves both neural and non-neural mechanisms, including structural alterations of muscles and surrounding fascia. Although botulinum toxin type A (BoNT-A) is widely used to treat focal spasticity, its effects on fascial tissue remain unclear. This retrospective observational study investigated longitudinal changes in fascial thickness after BoNT-A injection using ultrasonography. Thirty-seven patients with post-stroke spasticity were included. Fascial thickness was measured at baseline, 3 weeks, and 3 months after treatment in the injected muscle regions. Clinical outcomes were assessed using the Modified Ashworth Scale (MAS), Fugl-Meyer Assessment (FMA), and Brunnstrom stages. Repeated-measures analysis demonstrated significant reductions in fascial thickness across all evaluated regions. Pairwise comparisons showed significant decreases from baseline to week 3 and month 3, whereas continued reductions between week 3 and month 3 were observed only in the flexor digitorum superficialis fascia, the flexor digitorum superficialis-flexor digitorum profundus interfascial layer, and the deep brachialis fascia. MAS scores and upper extremity Fugl-Meyer scores improved significantly, whereas Brunnstrom stages remained unchanged. These findings suggest that the therapeutic effects of BoNT-A may extend beyond neural inhibition to include fascial remodeling. Ultrasonographic assessment of fascial thickness may represent a potential imaging biomarker for monitoring structural treatment responses in post-stroke spasticity.