--- - |2 Abstract This study investigated the mediation effects of social capital on the associations between the quality of relationship, intimacy in particular, with college friends on social media and loneliness during college‐to‐work transition of Korean college students. Longitudinal data were collected from three surveys with a 6‐month interval. The first wave of the survey was completed by 521 participants; 110 graduates participated in the second wave, and 175 in the third wave. Latent growth modeling analysis revealed that the linear growth function produced a better fit than the no‐growth models for intimacy on social media, social capital, and loneliness. While intimacy on social media and bonding capital increased during the three‐time interval, bridging capital and loneliness decreased. In addition, the slope of intimacy on social media was positively related to the slopes of bridging and bonding capital, both of which, in turn, had a negative association with the slope of loneliness. These findings indicate the beneficial role of the quality of relationship on social media in college‐to‐work transition. - Japanese Psychological Research, EarlyView.