Objective The aim of this study was to identify risk factors for obesity in childhood cancer survivors (CCSs). Methods The study included 3199 patients of the French Childhood Cancer Survivor Study cohort, with 303 patients with obesity who had returned the self‐questionnaire. Analyses were adjusted for social deprivation index and sex. Results CCSs were less likely to have obesity (9.5%; 95% CI: 8.5%–10.5%) than expected from the general French population rates (12.5%; p = 0.0001). Nevertheless, brain tumor survivors were significantly more likely to develop obesity than the general French population (p = 0.0001). Compared with patients who did not receive radiotherapy to the pituitary gland, those who received a dose >5 Gy had an increased risk of obesity: relative risk 1.9 (95% CI: 1.2–3.1), 2.5 (95% CI: 1.7–3.7), and 2.6 (95% CI: 1.6–4.3), respectively, for participants with 6 to 20 Gy, 20 to 40 Gy, and ≥40 Gy of radiation. Etoposide administration significantly increased the risk of obesity (relative risk 1.7; 95% CI: 1.1–2.6). High social deprivation index was also a risk factor, just like BMI at diagnosis. Conclusions Long‐term follow‐up of CCSs should include weight follow‐up during adulthood.