The decreasing areal extent of Cape May Peninsula lagoons in historical times and surfaceward-increasing concentrations of anthropogenically released heavy metals in lagoonal sediment suggest that the Atlantic coastal marshes of southern New Jersey are rapidly accumulating fine-grained sediment. Potential sources of sediment to this area include Pleistocene mud deposits on the continental shelf, fine sediment moving from north to south with the longshore drift, and Delaware Bay mud escaping the estuary on ebb tides. To determine the origin of the modern coastal muds of south New Jersey, sediment from each source was compared with suspended sediment entering Cape May Peninsula tidal inlets on flood tides. Although suspended sediment was considerably finer grained than material from other locations, the mud fraction of all sediment samples possessed a primary mode of ∼0.5 µm and a secondary mode of ∼8 µm. All samples exhibited great variation in mineralogy with grain size but possessed similar relative abundances of minerals within fractionated size classes. In general, feldspar dominated the silt fraction, with increasing quantities of illite, chlorite, and montmorillonite found in progressively finer sizes. On the basis of mineralogy, bottom sediment from northeast Delaware Bay, the Atlantic inner continental shelf, and mud from New Jersey beaches cannot be differentiated and ultimately may be derived from the same material—the eroding, Pleistocene-age Cape May Formation. The mineralogy of suspended sediment exiting Delaware Bay on ebb tides and entering Cape May Peninsula inlets on flood tides is identical, and it differs from bottom sediment by possessing slightly more feldspar and biotite and less chlorite. A consideration of the water circulation around Cape May Peninsula and evaluation of all available Landsat imagery of the area suggest that resuspension of northeast Delaware Bay bottom sediment, augmented by contributions from the Delaware River, provides most of the fine-grained sediment to the Atlantic coastal marshes of Cape May Peninsula.