The braces of today are constructed to correct the frontal plane deformity of idiopathic adolescent scoliosis (AIS). The Spinaposture brace© (Spinaposture Aps, Copenhagen, Denmark) is a soft-fabric brace for AIS and is designed to enhance rotational axial stability by inducing a sagittal plane kyphotic correction. This prospective observational study evaluated the brace in fifteen patients with AIS. The initial average CA was 16.8° (SD: 2.8). They were followed prospectively every 3 to 6 months during their brace usage until skeletal maturity of 25 months and at long-term follow-up of 44 months. In- and out-of-brace radiographs were performed in six subjects at inclusion. This resulted in an immediate in-brace correction of 25.3 percent in CA (14.3°→10.8°) and induced a kyphotic effect of 14.9 percent (40.8°→47.9°). The average in-brace improvement at first follow-up was 4.5° in CA, and the CA at skeletal maturity was 11° (SD: 7.4°) and long-term 12.0° (SD: 6.8°). In conclusion, the Spinaposture brace© had an immediate in-brace deformity correction and a thoracic kyphotic effect. At skeletal maturity, the deformities improved more than expected when compared to that of the natural history/observation and similar to that of other soft braces. No long-term deformity progression was seen. To substantiate these findings, stronger designed studies with additional subjects are needed.