Although dispositional optimism serves as a resource for psychological and physical well-being, very little research has explored how to promote child dispositional optimism within the family context. The authors evaluated paternal emotional expressiveness within the family as a mediator of the association between father's and child's dispositional optimism. Participants were 422 father (M age = 44.97 years, SD = 4.21 years)-child (M age = 11.75 years, SD = 0.77 years) dyads in South Korea. Results showed that fathers' dispositional optimism was positively linked to their expressiveness, which in turn was positively linked to their children's dispositional optimism. No significant gender or socioeconomic status differences were found. The findings have important implications for future research and for preventive interventions.