Modern Western diets, characterized by a substantial proportion of kilocalories derived from ultraprocessed foods (UPF), have been associated with systemic inflammation. This study examines the association between UPF consumption and inflammation, assessed through alterations in C-reactive protein (CRP) concentrations, among Brazilian adolescents. This is a cross-sectional study involving a sub-sample of 6,316 adolescents aged 12 to 17 years, participants in the Study of Cardiovascular Risk in Adolescents (ERICA), from seven capitals in Brazil. Dietary intake was assessed using a 24-hour recall, and foods were categorized based on their degree of processing according to the NOVA classification. UPF consumption was then divided into quartiles. For CRP evaluation, blood samples were collected after a 12-hour fasting period and categorized as >3 mg/L, indicating low-grade inflammation. Poisson regression models with robust variance were employed to assess the association between UPF consumption and high CRP concentrations. High UPF consumption (highest quartile, ≥ 44.9% kcal/day) was slightly associated with a higher prevalence of CRP after adjusting for potential confounders (PR = 1.039; 95% CI: 1.006; 1.073), compared to those in the lowest quartile of UPF consumption. However, when evaluating different groups of UPFs separately (such as sugary beverages, processed meats, and sweets), the previous association was no longer observed. These findings suggest a modest association between overall UPF consumption and early indicators of unhealthy low-grade inflammation in adolescents. Further experimental and cohort studies are necessary to clarify the role of UPF in inflammatory processes.