Research shows that performing positive activities, such as expressing gratitude and doing acts of kindness, boosts happiness. But do specific positive activities work equally well across cultures? Our study examined the role of culture–activity fit by testing two positive activities across two cultures. Participants from the United States (n = 250) and South Korea (n = 270) were randomly assigned to express gratitude, perform kind acts, or engage in a neutral activity for the first half of a 6-week positive activity intervention. Multilevel growth modeling analyses revealed that the effect of practicing gratitude or kindness was moderated by culture: U.S. participants increased in well-being (WB) from both activities, γ11 = 0.19, SE = 0.06, t(511) = 3.04, p = .0006; γ12= 0.11, SE = 0.06, t(511) = 1.73, p = .03 (compared with the control group), but South Korean participants benefited significantly less from practicing gratitude than did U.S. participants, γ13 = −0.24, SE = 0.07, t(511) = −3.36, p = .002. South Korean participants, however, showed similar increases in WB as did U.S. participants when performing kind acts, γ14 = −0.06, SE = 0.07, t(511) = −0.82, ns. Finally, although greater self-reported effort yielded significantly larger increases in WB for U.S. participants, the effect of effort was not as strong for South Korean participants. We posit that, due to their dialectical philosophical tradition, South Koreans might have been more prone to feel mixed emotions (e.g., indebtedness and gratitude) while engaging in the gratitude letter activity than did U.S. participants.