Purpose To compare the value of endorectal ultrasonography (US) with shear-wave elastography (SWE) in staging rectal tumors before surgery. Materials and Methods This prospective study was approved by the institutional review board and written informed consent was obtained. In a pilot cohort from April 2015 to January 2016, 70 patients with rectal adenocarcinomas and/or adenomas confirmed with histopathologic examination underwent both endorectal US and SWE. Tumor stiffness and three regions of reference backgrounds, as well as tumor stiffness ratios (SRs) versus these backgrounds, were analyzed. The optimal staging feature was selected by using receiver operating characteristic analysis, and the concordance rate with pathologic stage was analyzed and compared with endorectal US. The results were validated in an independent cohort of 30 patients from February 2016 to July 2016. Results In the pilot study, from rectal adenoma to stage T3 cancers, the tumor stiffness, stiffness of peritumoral fat, tumor SR versus distant perirectal fat, and tumor SR versus normal rectal wall were significantly increased (all P < .05, correlation coefficients between SWE features and pathologic T stages were 0.589-0.853). Receiver operating characteristic analysis of tumor staging demonstrated that tumor stiffness was the optimal feature with the highest area under the receiver operating characteristic curve (AUC = 0.991-1.000). The cutoff values of stage T1, T2, and T3 cancers were 26.9 kPa, 70.3 kPa, and 112.0 kPa, respectively. For SWE, the diagnostic concordance rate with pathologic stage (95.7%, weighted κ = 0.962) was higher than that of endorectal US (75.7%, weighted κ = 0.756). In the validation cohort, similar findings were revealed for diagnostic concordance rate (93.3% vs 76.7%; weighted κ = 0.927 vs 0.651) and diagnostic performance of tumor staging (AUC = 0.950-1.000 vs 0.700-0.833). Conclusion By using the feature of tumor stiffness at SWE, the accuracy of preoperative staging for rectal tumors was improved compared with endorectal US in the pilot study, but was not significantly different in the validation cohort, potentially due to small sample size. RSNA, 2017 Online supplemental material is available for this article.