Background: To determine the diagnostic performance of qualitative and quantitative shear wave elastography (SWE) and the optimal cutoff values of the quantitative SWE parameters in differentiating malignant from benign breast masses, and to evaluate the association between the quantitative SWE parameters and histological prognostic factors. Methods: A gray scale ultrasound and SWE were prospectively performed on a total of 244 breast masses (148 benign, and 96 malignant) in 228 consecutive patients before an ultrasound-guided needle biopsy. The qualitative SWE and quantitative SWE parameters (the mean elasticity, maximum elasticity, and elasticity ratio) were measured in each mass. The diagnostic performance of SWE and the optimal cutoff values of the quantitative SWE parameters were obtained. An association analysis of the parameters and histological prognostic factors was performed. Results: The malignant masses had a more heterogeneous pattern on the qualitative SWE than benign masses (P<0.001). The quantitative SWE parameters of the malignant masses were higher than those of the benign masses (P<0.001); the mean elasticity, maximum elasticity, and elasticity ratio of the benign masses were 19.73 kPa, 23.98 kPa, and 2.78, respectively; and the mean elasticity, maximum elasticity, and elasticity ratio of the malignant masses were 88.13 kPa, 98.48 kPa, and 10.64, respectively. The optimal cutoff value of the mean elasticity was 30 kPa, of the maximum elasticity was 36 kPa, and of the elasticity ratio was 4.5. The maximum elasticity had the highest AUC. Combining the three SWE parameters to differentiate between the malignant and benign masses increased the negative predictive value (NPV), which correctly downgraded 72.73% of BI-RADS category 4A masses to BI-RADS category 3. No statistically significant association was found between the quantitative SWE parameters and the tumor grading, tumor types, axillary lymph node statuses, or molecular subtypes of the breast cancers (P>0.05). Conclusions: The qualitative and quantitative SWE provided good diagnostic performance in differentiating malignant and benign masses. The maximum elasticity of the quantitative SWE parameters had the best diagnostic performance. Adding the three combined quantitative SWE parameters to the BI-RADS category 4A masses potentially downgraded them to BI-RADS category 3 and avoided unnecessary biopsies. No statistically significant association was found between the quantitative SWE parameters and the histological prognostic factors.