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Transitioning from a dyadic to a triadic family structure after childbirth requires substantial adaptation to maintain relationship harmony. We develop and validate the Harmony Scale for Postpartum Couples (HSPC) to assess harmony among first-time parents. The scale development proceeded in three phases. In Study 1, a conceptual framework was established, and experts evaluated content and face validity. In Study 2, item analysis and exploratory factor analysis (EFA) were conducted to refine items and construct a preliminary scale. In Study 3, psychometric properties were examined using data from 376 participants (188 couples) with infants aged 3-6 months. EFA identified 15 items across four factors:[Intimacy with partner], [Physical・Social Health], [Mental Health], and [Self-Efficacy in Parenting]. The overall Cronbach's α for the scale was .825, indicating good internal consistency. Construct validity was supported by confirmatory factor analysis indices (GFI = .908, AGFI = .869, CFI = .880, RMSEA = .083). The final 15-items, four-factors HSPC demonstrated reliable and valid performance for evaluating couple harmony during the early postpartum period. This concise instrument provides comprehensive assessment of couple harmony, supports early identification of relational difficulties, and can inform targeted interventions to promote healthy family functioning, parental well-being, and infant health. J. Med. Invest. 73 : 176-185, February, 2026.