Loneliness has previously been linked to cognitive and attentional bias, and such biases may have a detrimental impact on perceived scholastic self-beliefs. Little is known about the relationship in school-aged adolescents. The current study examined the association between loneliness and scholastic self-beliefs in a nationally representative Danish sample of adolescents (aged 11-, 13- and 15 years, n = 3815, collected in 2014 by the Health Behaviour in School-aged Children study (HBSC)). Through binary logistic regressions, results demonstrated that higher levels of loneliness, measured by a single item and a composite score, were associated with poorer self-reported achievement perception, higher feelings of school dissatisfaction, and greater feelings of school pressure. Results also suggested gender played a moderating role. The current study highlights the importance of loneliness for scholastic self-beliefs, and provides a novel insight by utilising distinct loneliness measures. The implications, in relation to research and practise, are discussed.