Abstract Purpose To evaluate the diagnostic performance of quantitative apparent diffusion coefficient (ADC) measurements, in the assessment of the therapeutic response to chemo-radiation therapy (CRT) in patients with locally advanced rectal cancer, by analyzing post CRT values of ADC, in relation to tumor regression grade (TRG) obtained by histopathologic evaluation of the rectal specimen. Methods This prospective study was approved by an Institutional Review Board, and informed consent was obtained from all patients. Thirty-one patients with locally advanced rectal cancer underwent pre and post CRT MR imaging at 1.5 T. ADC values were measured in regions of interest (ROIs) drawn independently by two radiologists, blinded to the pathology results, on three slices of the pre and post CRT DW-MR image sets with the corresponding T2 weighted images (T2WI) available for anatomic reference. The two readers’ measurements were compared for differences in ADC values, inter-observer variability (measured as the intraclass correlation coefficient; ICC) and the ADC distributions of responders vs non-responders. The diagnostic performance of ADC in the prediction of the response to CRT was evaluated by calculating the area under the ROC curve (AUC) and the optimal cut-off values. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were assessed. Results The two readers showed an overall strong agreement in measuring ADC values. For both readers, no differences in ADC pre-treatment measurements were observed between responders and non-responders. For reader 1, the post-CRT ADC and the ΔADC presented the higher AUC (0.823 and 0.803, respectively), while Δ%ADC provided the lower AUC value (0.682). The optimal cutoff point was 1.294 s/mm2 for post-CRT measures (sensitivity = 86.4%, specificity = 66.7%, PPV = 86.4%, NPV = 66.7%), 0.500 for ΔADC (sensitivity = 81.8%, specificity = 66.7%, PPV = 85.7%, NPV = 60.0%) and 59.3% for Δ%ADC (sensitivity = 63.4%, specificity = 66.7, PPV = 82.4%, NPV = 42.9%). Similar results were observed for reader 2, with better performance obtained with the ADC post-CRT (AUC of 0.833) and an optimal cut off of 1.277 × 10−3 s/mm2. Conclusion Post-CRT ADC measurements are reliable and reproducible and may be used as a non-invasive tool to evaluate response to therapy as post-CRT ADC values and ΔADC presented good diagnostic performance to select responder patients.