Hormetic models of stress resilience describe nonlinear relations for exposure to adversity and health outcomes, where exposure induces salutary changes up to a threshold, with changes becoming deleterious afterward. Here we apply a hormetic model of stress to reactivity to daily stressors, examining whether mental and physical health benefits arise from low-to-moderate reactivity but then decrease at higher levels. Data are from the second wave of the National Study of Daily Experiences (NSDE). Adults (N = 2,022; Mage = 58.61, SD = 12.12, age range: 35-86; 57% female) completed telephone interviews detailing their stressors and affect on eight consecutive evenings. A series of multilevel structural equation models estimated within-person associations between daily stressors and negative affect (i.e., stress reactivity), and between-person linear and quadratic effects of stress reactivity on mental and physical health outcomes (i.e., life satisfaction, psychological distress, and number of chronic conditions). Findings reveal a significant quadratic effect for each outcome, indicating a U-shaped pattern (inverse U for positively valenced life satisfaction), such that low and high levels of stress reactivity were associated with poorer health and well-being, whereas moderate levels of daily stress reactivity predicted better health outcomes. These findings suggest that individuals who display either very low- or very high-stress reactivity may benefit from interventions that target their emotion regulation skills and coping resources. (PsycInfo Database Record (c) 2024 APA, all rights reserved).