A maladaptive response to stress in individuals with high ADHD traits may be a key factor explaining co-occurring ADHD symptoms and internalising problems. The current study investigates whether between-person differences in ADHD traits are associated with differences in the within-person moment-to-moment coupling of stress and negative affect; and whether these can explain between-person differences in internalising problems (N = 262, median-age 20). Results of a dynamic structural equation model indicated that between-person differences in ADHD traits significantly moderated the daily life coupling between stress and negative affect. Further, higher ADHD traits were associated with stronger stress carry-over and higher mean levels of negative affect. Stress carry-over and mean levels of negative affect mediated the association between ADHD traits and internalising problems.