The magnetic structure of tetragonal UAu2Si2 was investigated by single-crystal neutron-diffraction experiments. Below T-N= 20 K it orders antiferromagnetically with a propagation vector of k = (2/3,0,0) and magnetic moments of uranium ions pointing along the tetragonal c axis. Weak signs of the presence of a ferromagnetic component of magnetic moment were traced out. Taking into account a group theory calculation and experimental results of magnetization and Si-29 nuclear magnetic resonance, the magnetic structure is determined to be a squared-up antiferromagnetic structure, with a stacking sequence (+ + -) of the ferromagnetic ac-plane sheets along the a axis. This result highlights similar magnetic correlations in UAu2Si2 and isostructural URu2Si2.