A large eddy simulation method based on a fully unstructured finite volume method was developed, and the unsteady aerodynamic response of a road vehicle subjected to transient crosswinds was investigated. The method was first validated on the 1/20-scale wind tunnel model at a static aerodynamic condition and showed good agreement in the surface pressure distributions. The method was then applied to two transient crosswind situations: a sinusoidal perturbation representing the typical length scale of atmospheric turbulence, and a stepwise crosswind velocity corresponding to a wind gust. Typical transient responses of the aerodynamic forces and moments such as phase shifting and undershoot or overshoot were observed, and their dependence on the frequency and amplitude of the input perturbation is discussed. As a result, the utility and validity of LES is demonstrated in the context that such transient aerodynamic forces are difficult to measure in the conventional wind tunnel measurements.