Background Diffusion MR imaging (DWI) is a widely available non-invasive non-contrast functional MR imaging technique with short acquisition time. It helps in the analysis of tissue characteristics based on the diffusion of water protons within the tissue. Quantitative assessment of a mass is possible by calculating its apparent diffusion coefficient (ADC) value which is inversely correlated with tissue cellularity. So, DWI has diagnostic potential to distinguish benign from malignant tumors because of the tendency of the latter to show lower ADC values and more restricted diffusion. The aim of our work is to evaluate the use of DWI and ADC value measurement in differentiation between benign and malignant mediastinal tumors. Results This study included 44 cases of mediastinal masses: 27 males and 17 females. The mean ADC value of malignant mediastinal lesions was significantly lower than the mean ADC value of benign mediastinal lesions, with mean ADC 1.39 ± 0.26 in benign mediastinal lesions and mean ADC 0.86 ± 0.35 in malignant mediastinal lesions. This study also revealed that the cut-off threshold of ADC value for the differentiation between malignant and benign lesions was 1.11 × 10-3 mm2/s, with an area under ROC curve of 0.93. The sensitivity and specificity of our cutoff ADC values were 90.9% and 100%, with 100% positive predictive value and 76.9% negative predictive value. Conclusion DWI with calculation of ADC value is functional MR imaging technique used in the analysis of tissue characteristics and quantitative assessment of a mediastinal mass. So, it can distinguish benign from malignant tumors because of the tendency of the malignant lesions to show more restricted diffusion and lower ADC values.