This study investigated the relationships among smartphone app use, social capital, and social isolation. It focused on two different smartphone apps--communication and social networking site (SNS) apps--and their effects on bonding and bridging social capital. Generational differences in smartphone use were also considered. Results from hierarchical regression analyses indicated that individuals' use of communication apps was helpful for increasing social capital and that this effect of using communication apps was stronger among those of the millennial generation than among older users. Moreover, bonding and bridging social capital was found to reduce individuals' social isolation significantly. These results imply the notable role of smartphone apps in reducing social isolation and improving the personal lives of individuals.