The current study examined age differences in the association between daily negative affect, average negative affect, and diurnal cortisol among participants from the National Study of Daily Experiences (N = 1,423; age range: 33–84 years). Across four consecutive days, participants reported the negative emotions they experienced and provided four saliva samples per day, from which cortisol was assayed. Results revealed that higher levels of average negative affect were associated with greater daily cortisol output (area-under-the-curve, with respect to ground), but only among the older participants in our sample. Higher levels of daily negative affect were also associated with elevated levels of bedtime cortisol, but only among older adults who, on average, reported lower levels of average negative affect. Findings support the theory of strength and vulnerability integration, and underscore the importance of age when examining associations between negative affective states and diurnal cortisol.