The present study aimed to determine the concurrent validity of ultrasound (US) measurement of the vastus lateralis muscle (VL) cross sectional area (CSA) having magnetic resonance imaging (MRI) as the gold-standard measurement, in a heterogeneous sample of participants. Thirty-one individuals (52.44 ± 16.37 yrs; 1.67 ± 0.11 m; 75.25 ± 13.82 kg) volunteered to participate in the study. All of the images were performed in the right leg. Imaging-fit technique (US) and computerized planimetry technique (US and MRI) were used to determine the VL CSA. The typical error of measurement (TE) was used to determine the concurrent validity of the US measurements. Our results demonstrated good validity of the US compared to the MRI measurements (TE = 0.37 cm²; CV = 1.75%). The Bland & Altman plot demonstrated bias of 0.07 ± 0.53 cm² and limits of agreement of 0.96 - 1.11 cm². Based on our TE, bias and limits of agreement, we concluded that the US image-fitting technique is valid to assess the VL CSA in a heterogeneous sample of participants. Thereby, the US can be used instead of the MRI to assess changes in skeletal muscle morphology.