Background: Cancer can have various psychosocial effects on children and young adults. Technology‐based psychosocial interventions are increasingly popular in paediatric oncology for psychosocial health. Objective: The objective of this study is to investigate the effectiveness of technology‐based psychosocial interventions applied to children, adolescent and young adults undergoing cancer treatment. Methods: Eight electronic databases including PubMed, Cochrane Library (including Cochrane Central Register of Controlled Trials [CENTRAL]), CINAHL Plus with Full Text, PsycINFO, MEDLINE, ProQuest, Science Direct and Scopus were searched to identify eligible randomised controlled trials from 1 January 2010 up to 9 April 2021. Search results were limited to 'children, adolescent and young adults' 6–24 years undergoing cancer treatment. Randomised controlled studies in which technology‐based psychosocial interventions were applied were examined. Of 4,927 articles, 12 articles met the inclusion criteria. Hedges' g effect size was computed for each study. Effect sizes and moderator variables were tested using random‐effects models. This study is registered with PROSPERO, number CRD42020177787. Results: The outcomes were anger, anxiety, coping, depression, distress, quality of life and resilience. Compared with standard care, technology‐based psychosocial interventions had a large effect and significantly on anger (g = −1.327, 95% CI: [−2.43, 0.22], anxiety (g = −0.89, 95%CI: [−1.34, −0.18], and depression (g = −1.05, 95% CI: [−1.65,−0.46]. Also, it was found that the duration of interventions had a significant moderator and effect size of the distress and resilience outcomes increased as the duration increased. Conclusion: This meta‐analysis demonstrated that technology‐based psychosocial interventions served as an effective intervention as it appears to have implications reducing anger, anxiety and depression in paediatric oncology. Nevertheless, randomised controlled studies with larger samples still needed to establish evidence‐based psycho‐oncological interventions for paediatric oncology. Future research should develop interventions more graduated by duration. [ABSTRACT FROM AUTHOR]