Background <p>Malnutrition is common in children with cerebral palsy (CP) due to feeding difficulties, altered body composition, and functional limitations, making an accurate nutritional assessment essential. The mid-upper arm circumference-for-age Z-score (MUACZ) is a practical screening tool, yet its diagnostic performance across childhood and adolescence remains insufficiently defined. The objective of this study was to assess the prevalence of undernutrition in children suffering from CP as well as the diagnostic performance of MUACZ in comparison with other anthropometric indicators across different age groups.</p> Methods <p>This cross-sectional study included 81 children with CP aged 2–18 years. Nutritional status was assessed using BMI-for-age (BAZ), height-for-age (HAZ), and MUACZ. Functional severity was classified using validated systems (GMFCS and EDACS). The diagnostic accuracy of MUACZ was evaluated with ROC analysis, and predictors of undernutrition were identified using multivariate logistic regression.</p> Results <p>Undernutrition (BAZ or HAZ ≤ − 2) affected 63% of the cohort, and its prevalence increased markedly with functional severity, reaching 88.6% in GMFCS level V compared with 10% in level I (<i>p</i> &lt; 0.001). MUACZ showed fair accuracy for detecting wasting (AUC = 0.757), with its strongest performance observed in children aged 5–10 years (AUC = 0.866). Older age and higher GMFCS levels were independent predictors of undernutrition.</p> Conclusion <p>Malnutrition in CP is closely linked to functional severity. MUACZ is simple, feasible, and may serve as a supportive screening tool to help identify undernutrition in children with CP.</p> Trial registration <p>ClinicalTrials.gov NCT07351695 (Retrospectively registered).</p>