PURPOSE Colorectal cancer (CRC) incidence rates are increasing among individuals < 50 years of age (early-onset CRC) globally with causes unknown. Racial/ethnic disparities in early-onset CRC have also grown more pronounced, because Black individuals have higher early-onset CRC incidence and poorer survival compared with White individuals. We describe the prevalence and burden of early-onset CRC among Africans in Nigeria and African Americans (AAs) in the United States. PATIENTS AND METHODS We identified Black individuals diagnosed with a first primary CRC ages 18 to 49 years between 1989 and 2017 at Ahmadu Bello University Teaching Hospital in Zaria, Nigeria (Nigerians), and in the United States (AAs) using the National Institutes of Health/National Cancer Institute’s SEER program of cancer registries. Multivariable logistic regression models were used to investigate clinical and demographic differences between Nigerians and AAs with early-onset CRC, adjusted for age, sex, tumor site, and histology. RESULTS A total of 5,019 Black individuals were diagnosed with early-onset CRC over the study period (379 Nigerians; 4,640 AAs). Overall, approximately one third of young Black patients were diagnosed with rectal tumors (35.8%). Nigerian individuals with early-onset CRC were eight-fold more likely to be diagnosed with rectal tumors (odds ratio [OR], 8.14; 95% CI, 6.23 to 10.62; P < .0001) and more likely to be diagnosed at younger ages (OR, 0.87; 95% CI, 0.86 to 0.89; P < .0001) compared with young African Americans in adjusted models. CONCLUSION Compared with AA individuals diagnosed with early-onset CRC, Nigerian individuals harbor distinct features of early-onset CRC. Additional investigation of the histopathologic and biologic heterogeneity of early-onset CRCs among Black individuals is critical for understanding racial disparities in susceptibility and outcomes, which may have implications for tailored early-onset CRC prevention, detection, and treatment strategies.