We investigated whether daily experiences of conflict with family and peers were associated with fluctuations in diurnal cortisol, and whether sleep buffers the associations between conflict and diurnal cortisol. A racially diverse sample of 370 adolescents (ages 11–18; 57.3% female) provided daily diaries for 5 days and saliva samples for 4 days. Hierarchical linear models tested how peer and family conflict were associated with diurnal cortisol (i.e., total cortisol output, cortisol slope, and cortisol awakening response) the next day, and whether these associations were moderated by sleep duration the previous night. When adolescents experienced peer conflict, they showed higher area under the curve (AUC) the next day if they had slept less the night prior to conflict, but relatively lower cortisol awakening response (CAR) and flatter cortisol slope the next day if they had slept more the night prior to conflict. When adolescents experienced family conflict, they also showed higher AUC the next day if they had slept less the night prior to conflict, but higher CAR the next day if they had slept more the night prior to conflict. Family conflict and sleep were not directly or interactively related to cortisol slope.