This experiment aimed to investigate the effects of seated posture regulation on children's psychological and physiological state and test performance. Thirty-eight boys (mean age: 12.3 ± 0.53 years) participated in both upright and normal posture conditions in a within-participants design. Participants completed a two-dimensional mood scale to measure psychological mood states and performed three tests (calculation, listening comprehension and word memory). Heart rate, as an index of physiological arousal state, was measured. Upright posture resulted in greater vitality and pleasure, as well as increased heart rate, compared with normal posture. Upright posture also led to significantly higher scores for calculation and listening comprehension tests. Moreover, increases in vitality level positively influenced increases in calculation and listening comprehension test scores. This study suggests that adopting an upright posture results in maintaining a positive psychological state and improving test performance of children.