Objectives Cross-sectional area (CSA) measurements of the neck musculature at the level of third cervical vertebra (C3) on CT scans are used to diagnose radiological sarcopenia, which is related to multiple adverse outcomes in head and neck cancer (HNC) patients. Alternatively, these assessments are performed with neck MRI, which has not been validated so far. For that, the objective was to evaluate whether skeletal muscle mass and sarcopenia can be assessed on neck MRI scans. Methods HNC patients were included between November 2014 and November 2018 from a prospective data-biobank. CSAs of the neck musculature at the C3 level were measured on CT (n = 125) and MRI neck scans (n = 92 on 1.5-T, n = 33 on 3-T). Measurements were converted into skeletal muscle index (SMI), and sarcopenia was defined (SMI <43.2 cm(2)/m(2)). Pearson correlation coefficients, Bland-Altman plots, McNemar test, Cohen's kappa coefficients, and interclass correlation coefficients (ICCs) were estimated. Results CT and MRI correlated highly on CSA and SMI (r = 0.958-0.998, p <0.001). The Bland-Altman plots showed a nihil mean Delta SMI (- 0.13-0.44 cm(2)/m(2)). There was no significant difference between CT and MRI in diagnosing sarcopenia (McNemar, p = 0.5-1.0). Agreement on sarcopenia diagnosis was good with kappa = 0.956-0.978 and kappa = 0.870-0.933, for 1.5-T and 3-T respectively. Observer ICCs in MRI were excellent. In general, T2-weighted images had the best correlation and agreement with CT. Conclusions Skeletal muscle mass and sarcopenia can interchangeably be assessed on CT and 1.5-T and 3-T MRI neck scans. This allows future clinical outcome assessment during treatment irrespective of used modality.