Background <p>Childhood maltreatment (CM) is a potent predictor of lifelong emotional and psychological difficulties. We investigated how CM affects intra- and interpersonal emotion regulation (ER) processes and explored the impact of these ER difficulties on daily life mood.</p> Methods <p> <?tk 1?>We explored the CM-ER association in two studies. Data and code are available at <a href="https://osf.io/cbkyj/">https://osf.io/cbkyj/</a>. Study 1 tested pre-registered hypotheses (<a href="https://osf.io/2kt35">https://osf.io/2kt35</a>) on the association between CM and self-reported ER difficulties in a web-based sample (<i>N</i> = 553). Study 2 used ecological momentary assessment data (<i>N</i> = 103) to examine how trait-level intra- and interpersonal ER difficulties predict momentary negative mood in pseudo-randomized daily assessments over seven days (3,973 observations), particularly in the context of momentary interpersonal stressors.</p> Results <p>We replicated a positive association between CM severity and intrapersonal ER difficulties and revealed differential effects of abuse versus neglect. Additionally, CM was associated with lower use of and more difficulties in interpersonal ER. In exploratory analyses, we found that intrapersonal ER difficulties predicted increased negative mood during interpersonal stressors, while interpersonal ER use showed no significant effects on momentary mood.</p> Conclusions <p>Our findings highlight a critical treatment target: intrapersonal ER impairments, which uniquely predict daily mood fluctuations beyond the effect of CM severity. Reduced use of interpersonal ER was also observed in individuals with more severe CM, which could be adaptive in certain environments where social support is unavailable or inconsistent. We emphasize the need to prioritize ER-focused interventions in clinical settings to address the enduring consequences of CM.</p>