Acquiring full control over a large number of diffraction orders can be strongly attractive in the case of realizing multifunctional devices such as multichannel reflectors. Recently, the concept of metagrating has been introduced, which enables obtaining the desired diffraction pattern through a sparse periodic array of engineered scatterers. In this Letter, for the first time, to the best of our knowledge, a tunable all-graphene multichannel meta-reflector is proposed for operating at terahertz (THz) frequencies. In the supercell level, the designed metagrating is composed of three graphene ribbons of different controllable chemical potentials which can be regarded as a five-channel THz meta-reflector. By choosing proper distribution of DC voltages feeding the ribbons, our design can realize different intriguing functionalities such as anomalous reflection, retroreflection, and three-channel power splitting within a single shared aperture and with high efficiency. This Letter paves the way toward designing highly efficient and tunable THz multichannel meta-reflectors with many potential applications in photonics and optoelectronics.