Background Loneliness in childhood has a wide range of negative consequences for well‐being and mental health later in life. This study reports time‐trends in children's loneliness and the association between children's loneliness and psychiatric symptoms over a 24‐year period. Methods Information on 3,749 eight‐year‐old Finnish‐speaking children born in 1981, 1991, 1997, and 2004 was gathered at four time points from the area covered by Turku University Hospital in southwest Finland. The actual numbers of participants at these time points were 986 (1989), 891 (1999), 930 (2005), and 942 (2013), with participation rates of 86%–95%. The study design and methods were similar at every time point. Information on children's loneliness and friendships was obtained from the children and also parents and teachers evaluated how many friends children had and their psychiatric symptoms. Results Approximately 20% of the children reported loneliness at each time point, 5% always felt lonely, and 25% wished they had more friends. Conduct and emotional problems, and hyperactivity were independently associated with loneliness in the multiple‐regression analysis. The strength of these associations remained at similar levels over the 24‐year study period. Living in a nonnuclear family, parents with a lower level of vocational education, and negative life events among the girls in the study were all associated with loneliness. Conclusions Loneliness was a common phenomenon in childhood, and no notable changes were found during the 24‐year study period. Psychiatric symptoms were strongly associated with loneliness. It is important to pay attention to children's loneliness and make it an integral part of school health care. Further epidemiological research is needed.