Nostalgia is a sentimental longing for one's past. We examined the effect of a 6-week, weekly nostalgia intervention on well-being (positive and negative affect, life satisfaction, subjective vitality, and eudaimonic well-being) over time. After 3 weeks, participants who engaged in nostalgic reflection had higher well-being than those who engaged in ordinary reflection. After 6 weeks, and at a 1-month follow-up, the positive effect of nostalgic reflection was reserved for those who were high on dispositional nostalgia (i.e., well-suited to the nostalgia intervention). However, at these time points, nostalgic reflection was associated with lower well-being among those particularly low on dispositional nostalgia. Across time points, nostalgic reflection was beneficial to the degree that it fostered social connectedness, meaning in life, and self-continuity, pointing to mechanisms that drive nostalgia's positive influence on well-being. In summary, weekly nostalgic reflection has temporary well-being benefits for most (out to 3 weeks) and, beyond that, is a matter of fit-beneficial or adverse to those especially high or low on dispositional nostalgia, respectively. (PsycInfo Database Record (c) 2022 APA, all rights reserved).