Considering happiness as fragile and fleeting rather than stable is negatively correlated with happiness. We predicted that the relationship between fragility of happiness beliefs and subjective happiness would be moderated by optimism. More specifically, we hypothesized that in individuals high on optimism the fragility beliefs would not be negatively related to well-being, whereas in individuals low on optimism, fragility beliefs would be negatively related to well-being. We tested this hypothesis in a sample of young South Koreans (N=246). The results supported our prediction, showing that fragility beliefs were significantly related to lowered happiness only when accompanied by low levels of optimism. These results suggest that conceptions of happiness may interact with other psychological variables to influence actual levels of happiness. •The relationship between fragility and subjective happiness was moderated by optimism.•At high and moderate levels of optimism, fragility was not related to happiness.•At low levels of optimism, fragility was negatively related to happiness.