We experimentally investigated gratitude’s impact on loneliness and health in older adults. Participants were assigned to a daily gratitude writing exercise (treatment group) or a control group. Self-reported loneliness and health (i.e., subjective well-being, subjective health, health symptoms) were measured daily over a 3-week period. In support of our hypotheses, within-person variability in gratitude predicted differences in loneliness and health. Furthermore, those in the treatment group showed stronger cumulative effects of gratitude on loneliness and health symptoms when aggregated across the 20-day study. Additionally, a series of conditional, multilevel indirect effect models found that loneliness acted as a mechanism for gratitude’s differential impact on subjective well-being and health symptoms across conditions. Taken together, this study provides initial evidence that a simple gratitude exercise can strengthen associations among daily gratitude and loneliness and, consequently, improve health, for older adults.