This longitudinal, within-subjects study examined whether adolescents' biological sensitivity to socioeconomic status (SES) for emerging social difficulties varied day to day. Diverse adolescents (N = 315; ages 11-18; 57% female; 25% Asian, 18% Latinx, 11% Black) provided daily diaries and saliva samples for 4 days. We measured biological sensitivity as daily fluctuations in diurnal cortisol slope, and SES as a principal component of family income and maternal education. A robust analysis of 1013 daily assessments revealed that youth from lower SES homes reported greater social difficulties "only" on days that they exhibited flatter diurnal cortisol slopes, and youth from higher SES homes reported fewer social difficulties on these days. SES was not associated with social difficulties on days that adolescents exhibited steeper, declining diurnal cortisol slopes. Findings support recent theory that risk and resilience are dynamic processes that change within individuals over time. For better and for worse, youth may be more biologically sensitive to their family socioeconomic environments on days that their diurnal cortisol rhythms are flattened.