BackgroundTrait‐level emotion regulation (ER) difficulties are associated with eating disorders (EDs) transdiagnostically. However, little research has examined whether within‐person fluctuations in ER longitudinally predict ED behaviors in daily life or the mechanisms of ER effects. Investigating daily ER could help us better understand why people experience ED behaviors at a given time. We examined whether day‐to‐day changes in adaptive (e.g., cognitive reappraisal) and maladaptive (e.g., rumination) ER longitudinally predicted core ED behaviors (binge eating, purging, dieting) and whether changes in affect mediated effects.MethodFemale participants (N = 688) ages 15–30 from the Michigan State University Twin Registry reported their adaptive and maladaptive ER use, negative affect (NA), positive affect (PA), binge eating, purging, and dieting on 49 consecutive days. Using structural equation modeling, we examined whether within‐person fluctuations in ER predicted same‐ and next‐day ED behaviors and whether changes in affect mediated longitudinal ER effects.ResultsGreater maladaptive ER predicted increased likelihood of same‐day binge eating and next‐day binge eating and purging. The association between maladaptive ER and next‐day binge eating and purging was mediated by increased next‐day NA. In contrast, dieting was more closely related to changes in PA. Adaptive ER did not predict reduced likelihood of any ED behavior.ConclusionsMaladaptive ER may longitudinally increase risk for binge eating and purging by amplifying NA. Interventions focused on decreasing maladaptive ER and subsequent NA might help disrupt binge eating‐purging cycles. Conversely, results add to evidence that PA fluctuations may play a unique role in maintaining restrictive behaviors.Public SignificanceLittle is known about how daily changes in emotion regulation may impact disordered eating. We found that maladaptive emotion regulation (e.g., rumination) was associated with a higher likelihood of binge eating and purging on the next day because it predicted increased next‐day negative affect. In contrast, dieting was more closely tied to fluctuations in positive affect. Targeting daily emotion regulation and affective processes may help disrupt cycles of disordered eating.