We report on the results of powder and single-crystal neutron diffraction to investigate the magnetic order in YbCo2Si2 below the Neel temperature T-N = 1.7 K in detail. Two different magnetically ordered phases can clearly be distinguished. At lowest temperatures a commensurate magnetic structure with a propagation vector k(1) = (0.25 0.25 1) is found, while the intermediate phase (T > 0.9 K) is characterized by an incommensurate magnetic structure with k(2) = (0.25 0.086 1). The magnetic structure in YbCo2Si2 is in marked contrast to all other known RCo2Si2 compounds (R = rare earth element) likely due to some itineracy of the Yb 4f states being responsible for the magnetism.