We report ultra-broadband, easily tuned, and highly efficient metamaterial terahertz wave retarders that are able to convert linear polarization into circular and elliptical polarization states. The broadband nature arises from the combination of the plasmonic effect and the property of polarization conversion. The dielectric response of the metamaterial microstructure and the angular phase dispersion of the grating coupler allow tuning of the flat phase differences from zero to 110 degree within the range of a few terahertz while keeping the magnitudes of the two orthogonally transmitted waves equal. In particular, a high degree of circular polarization (>0.99) can be achieved between 1.78 and 4.88 THz for a specific dielectric value of spacer material and angle of incidence. Our study opens new opportunities for manipulating the broadband polarization responses of terahertz waves, and hence enables implementation of novel functional devices based on metamaterials for terahertz wave imaging and spectroscopy.