The polarimetric information has been widely used to interpret the Synthetic Aperture Radar (SAR) scene. Hence, many coherent and incoherent target decompositions have been recently introduced to extract polarimetric parameters with a physical meaning. Nevertheless, for most of them, the reciprocity assumption is assumed. For a bistatic Polarimetric SAR (PolSAR) sensor, the cross-polarization terms of the scattering matrix are not equal in general. This paper presents a generalization of the Target Scattering Vector Model (TSVM) to the bistatic case. Five roll-invariant parameters are necessary for an unambiguous description of the target scattering mechanism: \alpha_s, \Phi_{\alpha_s}, \tau_1, \tau_2 and \mu. The scattering type magnitude \alpha_s and phase \Phi_{\alpha_s} contain information on the scattering type mechanism. The target helicity \tau_1 is a measure of the target symmetry. The target helicity \tau_2 contains information on the asymmetrical part of the scattering matrix, and \mu is the maximum amplitude return.