Objectives: The study aimed to evaluate the validity and reliability of the Turkish version of the General Self-Efficacy Scale in children and adolescents aged 8–18 undergoing cancer treatment. Methods: The sample of this methodological and descriptive study consisted of 128 children and adolescents between the ages of 8 and 18 in the pediatric hematology-oncology wards and outpatient units of a university hospital. The study was implemented in a university hospital in Turkey between March 2023 and November 2023. Data were collected through face-to-face interviews with children and adolescents attending the clinic or outpatient clinic. Data were collected using the information form and General Self-Efficacy Scale psychometric tests, Cronbach's alpha, Test-retest, validity and reliability analysis). Item-total score correlation was used. Results: In the exploratory factor analysis, Kaiser Meyer Olkin was p 0.77, for which Bartlett's test of sphericity was significant (p < 0.001). Exploratory factor analysis revealed a ten-item unidimensional structure with satisfactory representation. Item-total score correlations of the scale ranged between 0.32 and 0.52. In the confirmatory factor analysis, the model fit indices of this scale were obtained as follows: χ2/sd = 1.36, Goodness-of-Fit Index = 0.94, Comparative Fit Index = 0.92, Adjusted Goodness-of-Fit Index = 0.90, and Cronbach alpha value, indicating the internal consistency of the entire scale, was 0.76. Conclusions: The scale is a valid and reliable tool for assessing self-efficacy in children and adolescents between the ages of eight and eighteen undergoing cancer treatment. Practice implications: The study will provide precious information for experimental interventions and follow-up studies to assess the self-efficacy of children and adolescents with cancer.