Background and Aims The incidence and disease burden of colorectal cancer (CRC) in young adults were increasing. However, there was a dearth of advice on how to identify young population at risk for neoplastic colonic polyps (NCPs) and CRC. We aimed to identify risk factors for NCPs and CRC in young adults presenting with bloody stool. Methods A total of 1496 subjects younger than 40 years old who underwent colonoscopy due to bloody stool from 2005 to 2014 were enrolled in this retrospective study as the study group, and 1481 age‐matched and gender‐matched asymptomatic subjects who underwent colonoscopy for health checkup from 2011 to 2016 were enrolled as the control group at a tertiary center hospital. Results Multivariate analysis results showed that increasing age (odds ratio OR = 1.11, 95% confidence interval CI: 1.07–1.15, P < 0.001), higher body mass index (BMI) (OR = 1.07, 95%CI: 1.03–1.12, P = 0.001), diabetes mellitus (OR = 2.80, 95%CI: 1.06–7.42, P = 0.038), and positive family history of CRC (OR = 13.28, 95%CI: 5.70–30.97, P < 0.001) were identified as independent risk factors for NCPs in study group. The best cut‐off values by receiver operating characteristic curve for age and BMI were 32 years old and 24.8 kg/m2, respectively. More risk factors were associated with the higher risk for NCPs (OR = 2.17 every increasing one risk factor, P < 0.001). In the control group, no independent risk factors were identified. Conclusions Adults aged ≤ 40 years with bloody stool who had increasing age (> 32 years old), higher BMI (> 24.8 kg/m2), diabetes mellitus, and positive family history of CRC had a higher detection rate of NCPs and CRC.