Several methods have been developed over the last several years to analyze the mechanical properties of soft tissue. Elastography, for example, was proposed to evaluate soft tissue stiffness in an attempt to reduce the need for invasive procedures, such as breast biopsies; however, its qualitative nature and the fact that it is operator-dependent have proven to be limitations of the technique. Quantitative shearwave- based techniques have been proposed to obtain information about tissue stiffness independent of the operator. This paper describes shear wave dispersion magnetomotive ultrasound (SDMMUS), a new shear-wave-based method in which a viscoelastic medium labeled with iron oxide nanoparticles is displaced by an external tone burst magnetic field. As in magnetomotive ultrasound (MMUS), SDMMUS uses ultrasound to detect internal mechanical vibrations induced by the interaction between a magnetic field and magnetic nanoparticles. These vibrations generated shear waves that were evaluated to estimate the viscoelastic properties of tissue-mimicking phantoms. These phantoms were manufactured with different concentrations of gelatin and labeled with iron oxide nanoparticles. The elasticity and viscosity obtained with SDMMUS agreed well with the results obtained by traditional ultrasound-based transient elastography.