This paper probes the nanoparticle (NP) interaction with hydrogels using electroacoustic spectroscopy at MHz frequencies. We measured dynamic electrophoretic mobility spectra of silica NPs in polyacrylamide gels for a variety of NP sizes and gel concentrations. The spectra are exquisitely sensitive to NP entrapment, size, and charge as well as to gel rheology and gelation kinetics. For NPs that are large compared to the gel mesh size, many of these influences can be quantified using electrokinetic theory, which furnishes the apparent NP zeta-potential and a complex gel shear modulus at MHz frequencies. The methodology provides new insights into the NPhydrogel interaction, since it noninvasively probes the nanostructure and the combined influences of particle and gel properties. Electroacoustic spectroscopy may therefore be a valuable new tool for characterizing soft nanocompositesone that complements other noninvasive methods, such as bulk rheometry and microrheology.