Study Objectives: Emotional reactivity and regulation have been frequently proposed to explain the relationship between insomnia and depression. However, no longitudinal study has examined insomnia, depression, and emotional reactivity and regulation simultaneously and evaluated the mediating role of emotional reactivity and regulation. The current study investigated emotional reactivity and regulation as potential mediators of the insomnia-depression relationship in a three-wave longitudinal study over three months. Methods: Nine hundred and twenty-four participants with moderate to severe levels of insomnia were recruited (ages ranging from 16-69 years, 72.9% female). They filled out questionnaires that measured insomnia symptoms, depressive symptoms, emotional reactivity, and the use of emotion regulation strategies once per month for three months. Results: The results showed that depressive symptoms predicted insomnia symptoms (but not vice versa) via negative emotional reactivity, rumination, and catastrophizing. Conclusions: These novel findings highlighted the importance of targeting these aspects in emotional processes in assessing and treating individuals with insomnia and depression.