Importance Early-onset colorectal cancer (EOCRC) (diagnosed age &lt;50 years) incidence is increasing globally, in parallel with increased consumption of ultraprocessed foods (UPFs). The role of UPFs in early-onset colorectal neoplasia remains underexplored. Objective To evaluate the association between UPF consumption and risk of EOCRC precursors. Design, Setting, and Participants This prospective cohort study included participants of the Nurses’ Health Study II, an ongoing US prospective cohort of female registered nurses established in 1989. Participants were followed up from June 1, 1991, through June 1, 2015. Data were analyzed from October 2024 to July 2025. UPF intake, derived from food-frequency questionnaires administered every 4 years and classified using the Nova system, was modeled as quintiles of energy-adjusted servings per day. Of the nurses enrolled, those who had completed the baseline 1991 food-frequency questionnaire, undergone at least 1 lower endoscopy before age 50 years after baseline, had no history of cancer (except for nonmelanoma skin cancer) before endoscopy, and no colorectal polyp or inflammatory bowel disease were included. Main Outcomes and Measures Incidence of EOCRC precursors, including conventional adenomas and serrated lesions, confirmed via medical records and pathology reports. Multivariable logistic regression models with generalized estimating equations for clustered data were used to estimate adjusted odds ratios (AORs) and 95% CIs, accounting for known and putative risk factors. Results Among 29 105 female participants (mean [SD] age, 45.2 [4.5] years) over 24 years of follow-up, 1189 cases were documented of early-onset conventional adenomas and 1598 serrated lesions. UPFs provided 34.8% of total daily calories (median, 5.7 [IQR, 4.5-7.4] servings per day). Participants with higher UPF intake had an increased risk of early-onset conventional adenomas (highest vs lowest intake: AOR, 1.45; 95% CI, 1.19-1.77; overall P &lt; .001) but not serrated lesions (AOR, 1.04; 95% CI, 0.89-1.22; P = .48 for trend). Findings were consistent after further adjustment for body mass index, type 2 diabetes, dietary factors (fiber, folate, calcium, and vitamin D), and Alternative Healthy Eating Index–2010 score. Conclusions and Relevance In this study, higher UPF intake was associated with increased risk of early-onset colorectal conventional adenomas. These data highlight the important role of UPFs in early-onset colorectal tumorigenesis and support improving dietary quality as a strategy to mitigate the increasing burden of EOCRC.