Background: Sleep is critical to child development and health. Reaching the United Nations Sustainable Development Goal 3 regarding good health and well-being for all ages, sustainable and long-lasting interventions should be a priority for children. Purpose of the study: The aim was to evaluate the efficacy of weighted blankets on sleep and well-being among children with ADHD and sleeping difficulties. Methods: A randomised controlled trial (RCT) with a cross-over design included 94 children with ADHD and sleeping difficulties. Children were randomised to start with either a weighted blanket or a lighter control blanket. The children used the blankets during 4+4 weeks. Sleep was evaluated objectively with actigraphy and subjectively with child- and parent-rated questionnaires. Findings: Weighted blankets had a significant effect on total sleep time (mean diff: 8.05 min, p<0.05), sleep efficiency (mean diff: 0.83%, p<0.05) and parent-rated sleeping difficulties (mean diff: -1.05, p<0.05), but no significant effect on sleep initiation or child-rated insomnia severity or well-being (p>0.05) when evaluating weighted blankets compared to control blankets. Conclusion: This RCT showed that children with ADHD and sleep problems experienced improved sleep time and parent-rated sleep while using weighted blankets. Thus, weighted blankets are likely effective and an alternative to sleep medication. Good sleep is a significant determinant of health and well-being, especially for children with mental health problems. Consequences on child well-being may be more relevant in a long-term perspective and is yet to be determined.