We demonstrate that a driving ultrashort laser pulse undergoing filamentation can induce a remarkably large birefringence in Argon, resulting in an ultrafast “half-wave plate” for a copropagating non-filamenting probe beam. Such femtosecond birefringence, which originates from the difference between the nonlinear refractive indices induced by the filament on the axes parallel and orthogonal to its own polarization, opens the way to potential ultrafast Kerr-gates whose ultimate time-duration is only restricted by the duration of the driving pulse. We also show that the induced birefringence is transversely inhomogeneous, resulting from to the intensity profile of the driving pulse