Epidemiologic data indicate a significant increase in the incidence of colorectal cancer in younger populations in the past three decades. Moreover, recent evidence also demonstrates a similar trend in gastric, pancreatic, and biliary tract cancers. A majority of these early-onset cases are sporadic and lack hereditary or familial background, implying a potential key role for behavioral, lifestyle, nutritional, microbial, and environmental factors. This review explores the current data on early-onset gastrointestinal cancer, exploring the etiology, unique treatment considerations for this population, future challenges, as well as implications for research and practice. SIGNIFICANCE: The worrisome trend of an increasing incidence of early-onset gastrointestinal cancers appears to be correlated with nonhereditary etiologies in which behavioral, lifestyle, nutritional, microbial, and environmental factors, as well as host mechanisms, may play a key role. Further epidemiologic and pathogenetic research is urgently needed to better understand the underlying mechanisms and to develop preventive strategies and tailored early detection. Young patients with gastrointestinal cancer face unique challenges and unmet needs. These must be addressed in the future management of the disease to minimize treatment-related somatic morbidity and prevent psychosocial sequelae.