This study investigated: (a) the effect of emotion regulation (cognitive reappraisal and expressive suppression) on psychological distress and well‐being of couples in long‐term marriages, using longitudinal data; and (b) the influence of spouses' emotion regulation on psychological outcomes in the actor–partner interdependence model. In this longitudinal study, we conducted a baseline survey and a 1‐year follow‐up to analyze data of 66 couples (N = 132; duration of marriage, M = 36.82 years, SD = 6.83 years; husband's age, M = 64.50 years, SD = 4.05 years; and wife's age, M = 61.39 years, SD = 4.82 years) living in Japan. Emotion regulation, psychological well‐being, psychological distress, and various control variables (age, education level, duration of the marriage, and household income) were assessed. Hierarchical linear modeling with distinguishable dyads was used to determine actor–partner interdependence in the effect of emotion regulation on mental health. The actor effect was observed in both husbands and wives when baseline cognitive reappraisal was predicted to positively influence psychological well‐being in the follow‐up. However, this effect was not observed for expression suppression. Additionally, regarding psychological distress, there were no actor or partner effects in either reappraisal or suppression. Possible reasons for these findings, including cultural differences, are discussed in this study. [ABSTRACT FROM AUTHOR]