The aim of this study was to cross-sectionally explore the association of obesity with spinal posture and mobility, commonly associated with musculoskeletal problems, by comparing the spinal parameters between 90 obese and 109 normal-weight children and adolescents. A non-invasive electromechanical device, the Idiag M360 (Idiag, Fehraltorf, Switzerland), was used to measure the spinal parameters. An age-and-sex-adjusted two-way analysis of variance (ANOVA) was used to determine postural and mobility differences between the two groups. Children and adolescents with obesity had significantly greater thoracic kyphosis [difference between groups (Δ) = 13.00, 95% CI 10.10–15.80, p < 0.0001] and thoracic extension (Δ = 6.50, 95% CI 2.90–11.60, p = 0.005), as well as smaller mobility in thoracic flexion (Δ = 5.00, 95% CI 1.20–8.80, p = 0.01), thoracic lateral flexion (Δ = 17.70, 95% CI 11.60–23.80, p < 0.0001), lumbar flexion (Δ = 12.10, 95% CI 8.70–15.50, p < 0.0001), lumbar extension (Δ = 7.10, 95% CI 3.10–12.20, p = 0.003) and lumbar lateral flexion (Δ = 9.10, 95% CI 5.50–12.80, p < 0.0001) compared to the normal-weight children and adolescents. These findings provide important information about the characteristics of the spine in children and adolescents with obesity and unique insights into obesity-related mechanical challenges that the spine has to withstand and strategies designed to improve spinal mobility in this young population.