The San Gabriel, Chino, and San Bernardino sedimentary basins in Southern California amplify earthquake ground motions and prolong the duration of shaking due to the basins’ shape and low seismic velocities. In the event of a major earthquake rupture along the southern segment of the San Andreas fault, their connection and physical proximity to Los Angeles can produce a waveguide effect and amplify strong ground motions. Improved estimates of the shape and depth of the sediment-basement interface are needed for more accurate ground-shaking models. We obtain a three-dimensional basement map of the basins by integrating gravity and seismic measurements. The travel time of the sediment-basement P-to-s conversion, and the Bouguer gravity along 10 seismic lines, are combined to produce a linear relationship that is used to extend the 2D models to a 3D basin map. Basement depth is calculated using the predicted travel time constrained by gravity with an S-wave velocity model of the area. The model is further constrained by the basement depths from 17 boreholes. The basement map shows the south-central part of the San Gabriel basin is the deepest part and a significant gravity signature is associated with our interpretation of the Raymond fault. The Chino basin deepens towards the south and shallows northeastward. The San Bernardino basin, bounded by the San Jacinto fault (SJF) and San Andreas fault zone, deepens along the edge of the SJF. In addition, we demonstrate the benefit of using gravity data to aid in the interpretation of the sediment-basement interface in receiver functions.