The USArray seismic data have led to greatly improved images of upper-mantle structure under the contiguous United States, both from tomography to invert for volumetric heterogeneity and receiver-function studies to probe mantle discontinuities. Here I introduce an alternative approach to imaging mantle discontinuities that exploits long-period S-wave reflections seen in teleseisms. Analysis of USArray transverse-component data from 3250 earthquakes shows clear reflections off the 410- and 660-km discontinuities, which can be used to map the depth and brightness of these features. We also observe intriguing reflectors at shallower depths in many regions, including a negative polarity reflection that may delineate the top of the lithosphere-asthenosphere boundary (LAB) in some areas. We observe a dipping reflector in the west that aligns with the top of the high-velocity Farallon slab anomaly seen in some tomography models, but which also may be an artifact caused by near-surface scattering of incoming S waves.