Chronic exposure to daily stress can be harmful to mental health especially when individuals lack adaptive adjustment mechanisms. The present study aimed to investigate how the adaptive capacities in cognition and emotion as well as their neural signatures could moderate the stress reactivity in daily life. Seventy-five healthy participants aged 18-24 years participated in this study. We recorded brain activity using electroencephalography while participants were performing a conflict task and an emotion regulation task in the laboratory. Using the experience sampling method, participants were subsequently instructed to report their daily stress and daily affect on 14 consecutive days. Our results revealed that a larger adaptation effect in reaction times of the conflict task predicted a stronger negative affect in response to the stress of the same day. The adaptation effect in the N2 and P3 components elicited by the conflict task predicted a weaker influence of today's stress level on the next day's stress level, pointing to a better stress adaptation. However, emotion regulation capacities did not predict daily stress reactivity. Our data indicate that conflict adaption predicts two aspects of stress reactivity in daily life: how stress influences the negative affect that day, and how stress that day is related to stress the next day. These findings point to new avenues for early screening of stress-vulnerable populations, with implications for the prevention and intervention of stress-related mental disorders.