Background While the role of genetic predisposition in asthma and other allergic conditions is well established, the contribution of nutritional patterns is heterogeneous and has been demonstrated in cross-sectional studies but not in prospective cohorts. We examined the association between ultra-processed food (UPF) consumption and asthma and allergic diseases in a prospective cohort of Spanish children. Methods We analyzed data from 1,546 participants who were enrolled in the SENDO cohort between January 2015 and June 2024. Children aged 4–5 years were prospectively recruited. UPF consumption was assessed at baseline and updated at the 3-year follow-up. Information on asthma and allergic diseases was collected at baseline and updated annually during the follow-up. A final sample of 691 participants were classified into tertiles (T1, T2, T3) according to UPF consumption. Prevalent cases of asthma, allergic asthma (AA), atopic dermatitis (AD), food allergy (FA), aeroallergen sensitization (AS), and atopy cases were excluded from the analysis of the corresponding outcome in order to ensure that incident cases were included during the follow-up. The adjusted risk (and 95% Confidence Interval [CI]) of each outcome by tertile of UPF consumption was estimated using hierarchical regression modeling. In the main analyses, we calculated the adjusted hazard ratios (HRs) and 95% CI with survival analyses displayed using Nelson-Aalen plots. Results Our final sample included 691 children with a mean age of 4.86 years at baseline; 52.5% female. After a mean follow-up of 3.4 years, the adjusted risk for asthma in each tertile (T1, T2, T3) of UPF consumption was 2.6%, 9.9%, and 7.6% respectively (p for trend: 0.03). In the fully adjusted model of the survival analysis, children with greater UPF consumption (T2+T3) showed a significantly higher risk of asthma (HR 3.76; 95% CI 1.15-11.51, p=0.02) but not of AA (HR 2.14; 95% IC (0.55-8.31), p=0.27), nor other allergic outcomes compared with their peers in the lowest tertile (T1) of UPF consumption. Conclusion Higher UPF consumption maybe associated with an increased risk of developing asthma in school-age children.