Objective To evaluate the validity of the web-based version of the Previous Day Food Questionnaire Online for schoolchildren from the 2nd to 5th grades of elementary school. Methods Participants were 312 schoolchildren aged 7 to 12 years of a public school from the city of Florianópolis, Santa Catarina, Brazil. Validity was assessed by sensitivity, specificity, as well as by agreement rates (match, omission, and intrusion rates) of food items reported by children on the Previous Day Food Questionnaire Online, using direct observation of foods/beverages eaten during school meals (mid-morning snack or afternoon snack) on the previous day as the reference. Multivariate multinomial logistic regression analysis was used to evaluate the influence of participants’ characteristics on omission and intrusion rates. Results The results showed adequate sensitivity (67.7%) and specificity (95.2%). There were low omission and intrusion rates of 22.8% and 29.5%, respectively when all food items were analyzed. Pizza/hamburger showed the highest omission rate, whereas milk and milk products showed the highest intrusion rate. The participants who attended school in the afternoon shift presented a higher probability of intrusion compared to their peers who attended school in the morning. Conclusion The Previous Day Food Questionnaire Online possessed satisfactory validity for the assessment of food intake at the group level in schoolchildren from the 2nd to 5th grades of public school.