Background  Several studies have addressed mothers' perceptions of their children's weight status; however, there is no investigation on Portuguese children (a country with one of the highest levels of children's overweight and obesity in Europe). The aim of this study was to quantify maternal misclassification of child weight status in a sample of Portuguese children aged 9 to 12 years, according to gender, family income, and maternal weight status, education level and age.Methods  Data were collected in a school‐based study (school year 2009/2010) in northern Portugal with 499 urban children (236 girls; 47.3%). Body mass index was calculated from measurements of height and weight [body mass (kg)/height (m2)]. Mothers' perceptions of child's weight status, age, height and weight were accessed by a questionnaire. Children's age, gender and socio‐economic status were extracted from the schools' administrative record systems. Cohen's Kappa was used to analyse the misperceptions and the agreement between children's objectively measured weight status and mothers' perception of their child's weight status.Results  The prevalence of underweight, overweight and obesity in children was 4.6%, 25.5% and 6.4%, respectively. A proportion of 65.2% of underweight and 61.6% of overweight/obese children were misclassified by their mothers. For the majority of variables presented, the values of agreement were fair (k ranged from 0.257 to 0.486), but were statistically significant. Significant differences in the percentages of mothers who correctly classified their children's weight status were only found among the most educated in the overweight/obese group and among the normal‐weight mothers in the underweight group.Conclusions  Many mothers do not properly recognize their children's weight status and frequently underestimate their children's body size.