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<b>Background:</b> Postpartum mental health challenges are increasingly recognized as a major public health concern, particularly during the early months after childbirth when emotional vulnerability is heightened. Although aerobic exercise is known to be associated with mood improvement, few large-scale studies have examined the acute psychological effects of a single exercise session in postpartum women, especially in contexts where environmental barriers restrict opportunities for physical activity. <b>Methods:</b> A total of 628 postpartum mothers (2-6 months postpartum) residing in a snowbound region of northern Japan participated in a single-session indoor bounce-based aerobic exercise program. Mood states were assessed immediately before and after the session using the validated Japanese short-form Profile of Mood States (POMS). Open-ended comments were also collected and analyzed thematically. <b>Results:</b> Paired analyses demonstrated significant acute improvements in key mood domains. Tension-Anxiety decreased (mean difference -7.91, 95% CI [-8.82, -7.00]; d = -0.68), and Anger-Hostility decreased (-4.61, 95% CI [-5.40, -3.82]; d = -0.45), while Vigor increased (11.82, 95% CI [10.47, 13.17]; d = 0.69) (all <i>p</i> < 0.001). In contrast, Depression-Dejection showed no significant change (-0.33, 95% CI [-1.30, 0.64]; <i>p</i> = 0.613). <b>Conclusions:</b> A single session of indoor bounce-based aerobic exercise was associated with immediate improvements in emotional well-being among postpartum women, particularly in anxiety-related and activation-related mood states. Because this exercise modality can be performed safely at home and is well suited to regions where outdoor physical activity is restricted, it may represent an accessible strategy for supporting postpartum mental health. Future research should examine longitudinal effects, controlled designs, and optimal program frequency to determine sustained benefits.