The child's spine shows changes in posture and balance of its curvatures during growth and musculoskeletal spinal conditions are likely to develop, such as back pain (BP). The purposes of this study were (a) to describe the sagittal spinal alignment in a relaxed standing position and (b) to analyze its association with BP among 8 to 12-year-old children. This was a cross-sectional study. A total of 731 elementary schoolchildren (379 girls and 352 boys), from 16 Spanish schools, participated. An unilevel inclinometer was used to quantify the sagittal spinal curvatures (thoracic and lumbar) in a relaxed standing position. Sagittal spinal morphotype was analyzed by sex, age, weight, height, and BMI. Children's parents or legal guardians filled in a questionnaire according to the children's responses about the BP suffered in the previous week and the preceding year. The mean angular value of thoracic kyphosis was 36.08 ± 8.99∘ and significantly higher in males than in females (p= 0.036). In contrast, the mean value of lumbar lordosis was 32.11 ± 7.46∘, being higher in females than in men (p< 0.01). The thoracic curve tends to increase by age (p= 0.003). Children who had low back pain (LBP) in the previous week had a significantly greater lumbar curve (35.88 ± 8.20∘) than those who did not have LBP in the preceding week (32.24 ± 7.30∘). The angle for lumbar curvature was a small predictor of LBP occurrence in the assessed children (OR = 1.082 small; 95% CI = 1.009-1.160, p= 0.028). To conclude, almost 3/4 of the students were classified with normal thoracic kyphosis; however, 27.36% of the students had thoracic hyperkyphosis. Lumbar hyperlordosis was identified in 9.05% of the students, and was 2.5 times more frequent in girls. The lumbar curvature was a small predictor of LBP occurrence in the assessed children and the angle of lumbar curvature that most accurately identified individuals at risk of developing LBP was determined to be 33∘. The results of this study indicate the need to assess sagittal spinal curvatures at school during development ages.