A product-formula approach is used to stabilize 3D FDTD in a dispersive anisotropic magnetized plasma medium. The whole FDTD algorithm in a time step is split into three substeps, in two of which the complex medium properties are embedded. Another substep is an unconditionally stable FDTD (US-FDTD) in free space, where a locally one-dimensional (LOD) scheme is used here. The accuracy and stability of the proposed method are verified through simulation of wave propagation in a microwave cavity partially filled with a magnetized plasma slab.