This study aimed to examine in a longitudinal cohort design whether social media use among adolescents is related to symptoms of social anxiety, depressed mood, and physical symptoms of anxiety over time. As part of the LIFECOURSE study of risk and protective factors for healthy adolescent development, three waves of school-based surveys of adolescents born in Iceland in 2004 were analyzed. Of the 3914 eligible adolescents, 2378 gave informed consent. Complete responses for this study were collected from 2211 students at the first wave, with 2052 responding roughly 12 months later, and 2097 responding in year 3. Linear mixed-effects models were used to analyze time spent on social media in relation to psychological distress over time. More time spent on social media was weakly but significantly associated with increased symptoms of depressed mood, social anxiety and symptoms of physical anxiety over time. However, the effect size of these relationships suggest they may not be of clinical relevance. The relationship between time spent on social media and symptoms of depressed mood and physical symptoms of anxiety grew stronger over time, although it is not known if this relationship is causal. The relationship between time spent on social media and all outcomes of psychological distress were stronger for girls than boys and increased social media use had a positive relationship with symptoms of depressed mood over time. The relationships found in this study were relatively small and future studies need to focus on the clinical and public health significance of these effects.