We propose analytical expressions to determine the effective constitutive parameters of a planar bianisotropic metamaterial from scattering parameters in the terahertz region. In our retrieval method, the transmission and reflection coefficients in only one wave propagation direction are applied. Considering the nonsymmetry of planar metamaterials in the wave propagation direction, the effective refractive index and the impedance should be obtained by a modified S parameters retrieval process. The effective parameters of the permittivity, permeability, and magnetoelectric coupling coefficient of planar bianisotropic metamaterials can be retrieved by derived equations. Specifically, the constitutive parameters for different planar metamaterials, among which two are isotropic and the other two are bianisotropic metamaterials, are determined. The intrinsic differences between the normal planar metamaterials and the bianisotropic metamaterials are evidently illustrated. The phenomenon including electric coupling to magnetic resonance and only electric response in the transmission spectrum is confirmed by retrieval effective permittivity and permeability.