In the present study, the formability of an aluminum 6061 hollow-shaft gear produced by the radial extrusion process using a lead pressure-transmitting medium was investigated. In this method, a lead was placed inside the hollow shaft, and by applying a compressive load resulting in improved material flow and reduced stress concentration in critical regions. The required die set was designed and manufactured, and experimental tests were carried out using a hydraulic press. To achieve a more comprehensive investigation of the process, numerical simulation based on the finite element method was performed using DEFORM software, and the effects of key parameters including friction coefficient, gear height, and hollow shaft wall thickness on forming force, material flow pattern, die filling, and strain distribution were analyzed. The obtained results indicated that increasing the friction coefficient and decreasing the gear height led to an increase in the required forming force and a reduction in material flow velocity, whereas reducing the hollow shaft wall thickness decreased the forming force without having a significant effect on the material flow velocity. Furthermore, the good agreement between the numerical simulation results and the experimental data confirmed the reliability of the finite element method in predicting process behavior. Overall, the results of this study demonstrate that the use of a pressure-transmitting medium is an efficient approach for manufacturing aluminum 6061 hollow-shaft gears with desirable geometric quality and without major defects.