The associations between body fatness at a young age (childhood, adolescence and young adulthood; age ≤ 30 years) and diffuse large B‐cell lymphoma (DLBCL), oesophageal adenocarcinoma, gastric cardia cancer, hepatocellular carcinoma, multiple myeloma, pancreatic cancer, renal cell cancer and thyroid cancer remain inconclusive. We performed a comprehensive systematic literature review and meta‐analysis of observational studies to clarify the associations between body fatness at a young age and the risks of these cancers. PubMed and Web of Science databases were searched for relevant observational studies. Fifty‐six articles yielded data on 27,559 cancer cases, including 3,170 DLBCL, 1,491 oesophageal adenocarcinoma, 1,103 gastric cardia cancer, 1,067 hepatocellular carcinoma, 3,090 multiple myeloma, 7,220 pancreatic cancer, 6,212 renal cell cancer and 4,206 thyroid cancer cases. Each 5 kg m⁻² increase in body mass index at a young age was positively associated with DLBCL (relative risk [RR] 1.21, 95% confidence interval [CI] 1.09, 1.35), oesophageal adenocarcinoma (RR 1.88, 95% CI 1.37, 2.57), gastric cardia cancer (RR 1.59, 95% CI 1.15, 2.21), hepatocellular carcinoma (RR 1.31, 95% CI 1.13, 1.51), multiple myeloma (RR 1.23, 95% CI 1.15, 1.30), pancreatic cancer (RR 1.17, 95% CI 1.11, 1.24), renal cell cancer (RR 1.22, 95% CI 1.16, 1.28) and thyroid cancer (RR 1.12, 95% CI 1.07, 1.17). In summary, higher body fatness at a young age increases the risks of developing various types of cancer later in life. Prevention of overweight and obesity in children, adolescents and young adults should therefore be emphasized to reverse the obesity epidemic and thereby avoid further increases in the burden of cancer attributed to excess body fatness.