We propose an inversion model for retrieving the root-zone soil moisture profile using the P-band radar observations and validate it using data collected during the Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) mission. The forward model utilized in this article uses an existing small perturbation method (SPM) to calculate the electromagnetic wave scattered from multilayered rough surface dielectric structures. To solve the inverse problem, which is formulated as a least-squares problem, we utilize a novel global optimization technique that takes the advantage of the speed of a local optimization method while ensuring convergence to the global minimum. We present the numerical results to demonstrate significant improvements in the inversion results and computational speed compared to the previously used AirMOSS baseline inversion algorithm. The presented method is validated with in situ soil moisture measurements. Finally, by utilizing a closed-form analytical solution to Richards’ equation, we demonstrate improvements in the accuracy of our soil moisture retrieval results.