Colorectal cancers (CRCs) are rare in adolescents and adults 25 years or younger. We analyzed clinical, pathology, and molecular features of colorectal tumors from adolescents and young adults in an effort to improve genetic counselling, surveillance, and ultimately treatment and outcomes.We analyzed clinical data and molecular and genetic features of colorectal tumor tissues from 139 adolescents or young adults (25 years or younger; median age, 23 years; 58% male), collected from 2000 through 2017; tumor tissues and clinical data were obtained from the nationwide network and registry of histo- and cytopathology and the Netherlands Cancer Registry, respectively. DNA samples from tumors were analyzed for microsatellite instability, mutations in 56 genes, and genome-wide somatic copy number aberrations.Mucinous and/or signet ring cell components were observed in 33% of tumor samples. A genetic tumor-risk syndrome was confirmed for 39% of cases. Factors associated with shorter survival time included younger age at diagnosis, signet ring cell carcinoma, the absence of a genetic tumor-risk syndrome, and diagnosis at an advanced stage of disease. Compared with colorectal tumors from patients 60 years or older in the Cancer Genome Atlas, higher proportions of tumors from adolescents or young adults were microsatellite stable with nearly diploid genomes, or contained somatic mutations in TP53 and POLE, whereas lower proportions contained mutations in APC.We found clinical, molecular, and genetic features of CRCs in adolescents or young adults to differ from those of patients older than 60 years. In 39% of patients a genetic tumor-risk syndrome was identified. These findings provide insight into the pathogenesis of CRC in young patients and suggest new strategies for clinical management. Performing genetic and molecular analyses for every individual diagnosed with CRC at age 25 years or younger would aid in this optimization.