|The current study examines cross-national and age group differences in self-rated happiness, perceived neighborhood conditions and their linkages, with recent surveys representative of the national population in Japan and Korea. We find the adjusted level of happiness higher among young Japanese, but lower in the case of young Koreans, when compared to those of the older generations in each society. Japanese twenties and thirties show higher overall satisfaction about walkable residential areas in terms of neighborhood amenities and safety than Korean counterparts, but not as to neighbor relationships. In addition to intrapersonal factors, both socio-economic status and perceived qualities of neighborhood are significantly associated with happiness of Japanese across the life stage, whereas perceived neighborhood conditions do not contribute to happy lives of young Koreans, suggesting greater within-country group differences in Korea. The overall patterns in the key determinants of happiness are similar between young adults in both countries, but cross-national differences are observed in the effects of health status, self-efficacy, and neighborhood safety. Regarding neighbor relationships, age is a stronger moderator of buffering its positive effect on happiness in Korea, with young Koreans apparently less happy in neighborhood areas of higher bonding social capital. Discussing our findings as informed by further evidence, the present study considers divergent pathways to happiness of young adults in these two East Asian societies known to be similar in status-centered culture, collectivistic norms, and a wider institutional context.