<p>Stress is a major risk factor for mental health and well-being, yet few studies have examined its dynamic associations with hedonic and eudaimonic well-being from a within-person, dynamic perspective. While self-worth is considered an essential internal resource for sustaining well-being, it remains unclear whether it is involved in the momentary associations between stress and well-being at the momentary level. In addition, although grandiose narcissism has been proposed as a potential psychological buffer in stressful contexts, its protective function remains debated, with its efficacy potentially varying across temporal scales. In light of these unresolved questions, the present study employed a 10-day experience sampling design with four assessments per day among 166 participants and used dynamic structural equation modeling to test the dynamic relations between momentary stress and two forms of well-being, as well as the roles of momentary self-worth and grandiose narcissism trait. The results showed that (1) momentary stress significantly and negatively predicted subsequent hedonic well-being, but did not directly predict subsequent eudaimonic well-being; (2) momentary self-worth mediated the links between momentary stress and subsequent hedonic and eudaimonic well-being; and (3) grandiose narcissism trait moderated the association between momentary stress and subsequent self-worth, but this moderating effect was not observed when the time interval increased or at the between-person level. Taken together, these findings suggest that self-worth may serve as an important intermediate process linking stress and well-being, whereas the short-term buffering role of grandiose narcissism appears to be limited.</p>