A study of the antinucleon-nucleon interaction within chiral effective theory is presented. This novel approach suggested by Weinberg for investigating nucleon-nucleon interaction can be adapted straightforwardly to the antinucleon-system. The antinucleon-nucleon potential is derived up to next-to-next-to-next-order in the chiral expansion. The low-energy constants associated with arising contact interactions are fixed by a fit to phase shifts and inelasticities provided by a recently published phase-shift analysis of antiproton-proton scattering data. Theachieved description of the antinucleon-nucleon amplitudes is excellent and of a qualitycomparable to the one found in case of the nucleon-nucleon interaction at the same order.