Individuals feel lonely when they perceive a discrepancy between their desired and their actually experienced quantity and quality of social relationships. Prior research has demonstrated the importance of loneliness for various health-related aspects. In the present article, we extend the existing literature on loneliness by investigating its role for predicting personality traits and their development from late adolescence to early midlife. Using data from a representative German sample (N = 12,402) sampling individuals from three different birth cohorts, we found loneliness to predict the levels of all Big Five traits except openness five years later. The effects of loneliness on the development of neuroticism and extraversion reached statistical significance but were only marginal in terms of effect size. Furthermore, we found that a self-regulatory focus geared to the prevention of negative events mediated the effects of loneliness on later levels of the Big Five.