The association between the age at onset of metabolic syndrome and cancer risk remains unknown. This study explored the association between age at metabolic syndrome onset and the risk of overall and site-specific cancer incidence.This study included 31,688 participants with new-onset metabolic syndrome and 31,688 participants matched according to sex, age (±1 y), and examination year among the 179,328 participants who underwent Kailuan health examinations from 2006 to 2017 in Tangshan, China. Weighted Cox regression was used to calculate the hazard ratios and 95% confidence intervals of new-onset metabolic syndrome for overall and site-specific cancer incidence across age groups. Population-attributable risk proportions were used to estimate the number of cases that could be prevented by eliminating the risk factors from the population.During an average follow-up period of 10.22 y, we identified 2,710 cases of cancer and 4,218 deaths that occurred before the diagnosis of cancer. With an increase in metabolic syndrome onset age, the hazards of overall cancer incidence were gradually attenuated. The average hazard ratios (95% confidence intervals) of overall cancer were 1.94 (1.25-2.99) for metabolic syndrome onset age <45 year old, 1.41 (1.15-1.71) for age 45-54 years old, 1.38 (1.11-1.73) for age 55-64 years old, and 1.07 (0.89-1.28) for age ≥65 years old, respectively (p for interaction = 0.005). Similar results were obtained for colorectal, liver, and breast cancers in the site-specific analysis.New-onset metabolic syndrome was associated with a higher risk of overall cancer and incidence of several types of cancer, and the associations were stronger with a younger age of onset.Kailuan Study, ChiCTR2000029767 (Registered February 12, 2020, https://www.chictr.org.cn/showprojEN.html?proj=48316).Copyright © 2023. Published by Elsevier Ltd.