Near‐surface seismic velocity structure plays a critical role in ground motion amplification during large earthquakes. In particular, the local Vp/Vs ratio strongly influences the amplitude of Rayleigh waves. Previous studies have separately imaged 3D seismic velocity and Vp/Vs ratio at seismogenic depth, but lack regional coverage and/or fail to constrain the shallowest structure. Here, we combine three datasets with complementary sensitivity in a Bayesian joint inversion for shallow crustal shear velocity and near‐surface Vp/Vs ratio across Southern California. Receiver functions–including with an apparent delayed initial peak in sedimentary basins, and long considered a nuisance in receiver function imaging studies–highly correlate with short‐period Rayleigh wave ellipticity measurements and require the inclusion of a Vp/Vs parameter. The updated model includes near‐surface low shear velocity more in line with geotechnical layer estimates, and generally lower than expected Vp/Vs outside the basins suggesting widespread shallow fracturing and/or groundwater undersaturation. Plain Language Summary: Our study focuses on finding a new model to accurately image the near‐surface and upper crust of Southern California, as this structure is critical in amplification of ground motion during large earthquakes. To accomplish this, we uniquely combine seismic data from hundreds of Southern California stations to retrieve surface waves and body waves, including from basins where body‐wave data is typically discarded for being too great a nuisance. By employing a revolutionary processing technique after obtaining these datasets, we are able to test the robustness of our model by quantifying its uncertainty and sensitivity. Our new model includes fluid‐saturated sediments in the Los Angeles, Salton Trough, Central Valley, and Ventura basins. Additionally, we image hard, crystalline rock in the Peninsular and Sierra Nevada Mountain Ranges, and see evidence for rock origins in marine or continental environments, respectively. We are also able to see changes in structure across major faults, and areas of high‐fracturing. Outside of major basins, our overall results suggest widespread shallow fracturing and/or groundwater undersaturation. Key Points: We present near‐surface Vp/Vs and upper crust shear velocity models of Southern California via Markov Chain Monte Carlo joint inversionJoint inversion uses receiver functions, and ambient‐noise derived, short‐period, Rayleigh‐wave phase velocity and ellipticity (H/V) dataVp/Vs ratios fit fluid‐saturated sediments in major basins and are otherwise low, implying fracturing and/or groundwater undersaturation [ABSTRACT FROM AUTHOR]