INTRODUCTION: Despite evidence highlighting the dynamic nature of ethnic-racial identity (ERI) development and the common occurrence of discriminatory experiences, many studies treat these constructs as static and equivalent across individuals. Drawing upon the Phenomenological Variant of Ecological Systems Theory (PVEST), this study examined the within-person covariations between ethnic-racial affect (individuals' positive feelings regarding their ethnic-racial background) and discrimination-based stress, and whether these relations were moderated by average affect and average stress. METHOD: This study employed an intensive longitudinal design with 771 observations nested within 133 participants (Mage = 16.07, SD = 0.67), 52.3% were girls and ~93.3% were African American from Chicago, Illinois. RESULTS: Results from the multilevel model analysis revealed that within-person fluctuations in ethnic-racial affect were predicted by discrimination-based stress and that these fluctuations were person-specific. Furthermore, findings from this study also showed that the within-person fluctuations in ethnic-racial affect in relation to stress from discrimination were weaker for those with higher average affect and stronger for those with higher average stress. CONCLUSIONS: This study highlights the dynamic and situational nature of developmental processes by emphasizing the within-person fluctuations and person-specificity. These findings highlight the importance of developing and delivering interventions and programs that promote positive ethnic-racial affect to mitigate the negative impact of discrimination. These initiatives should be offered consistently and tailored to address individuals' specific needs to maximize their effectiveness.