<p>Irritability is a multidimensional construct prevalent in psychiatric disorders, theoretically comprised of tonic (persistent irritable mood) and phasic irritability (reactive outbursts) dimensions. However, few studies have examined how these distinct aspects relate to rumination among adults. Our current study applies a daily diary design to examine intraindividual covariation between irritable mood and reactive aggression and rumination among adults. The current study includes 100 adults with self-reported lifetime history of depression. Participants completed a baseline survey and up to 18 days of daily diary that assessed brooding rumination, irritable mood, and reactive aggression. Results from mixed effects models revealed that day-to-day fluctuations in rumination (time-varying) and enduring rumination tendency (time-invariant) were associated with both irritable mood and reactive aggression. While time-varying rumination was associated with both outcomes, a significant interaction was found for reactive aggression: individuals with higher enduring rumination tendency showed stronger associations between day-to-day fluctuations in rumination and reactive aggression. No such interaction was observed for irritable mood. Our findings suggest day-to-day fluctuations in rumination and enduring rumination tendency as a shared cognitive basis underlying both irritable mood and reactive aggression in adults. Practitioners treating adult irritability should consider interventions that would address both ruminative thinking style and momentary state rumination.</p>