Thorough and accurate models of modern sand sources to the Santa Monica Basin, offshore southern California, are needed to facilitate the interpretation of sediment supply to the Hueneme–Mugu Fan, the largest submarine fan within the basin. Bathymetry and near-seafloor seismic-reflection isopach mapping of basin fill indicate that dominant sources of sand are canyons/channels that enter the basin from the northwest, which are likely fed via longshore drift by the Santa Clara River and Calleguas Creek. Sand within Calleguas Creek varies in composition across its drainage basin, ranging from more quartzofeldspathic in the northeast to more volcaniclastic in the southwest, ultimately producing sand with a compositional fingerprint distinct from that of the Santa Clara River and Santa Monica Mountains. The abundance of volcanic material and lack of metamorphic grains in downstream Calleguas Creek sand stand in stark contrast to the Santa Clara River’s relatively abundant metamorphic lithic fragments. In addition, Calleguas Creek sand can be further differentiated from sand derived from both the Santa Clara River and Santa Monica Mountains because both of these other sources have much higher proportions of plagioclase. The composition of late Pleistocene (<60 ka) sandy turbidites at Ocean Drilling Program Site 1015 on the distal Hueneme–Mugu submarine fan validates Calleguas Creek’s contribution of sandy sediment to this site in the Santa Monica Basin: out of 14 samples, four samples show compositions similar to the Santa Clara River sand, whereas five are similar to Calleguas Creek sand, and six exhibit mixed compositions. There is no indication of input to this distal environment from the southern Santa Monica Mountains. Trends in sand composition within the Santa Monica Basin can be related to alternating and/or mixing of sediment sources, possibly related to sea-level change, as well as frequency of floods/storms, earthquakes, and other destabilizing processes affecting offshore shelf-to-slope regions.