The crystal structure and magnetic ordering of the novel orthorhombic compound ErGe3 of RGe3-type structure (Cmcm) was studied by neutron powder diffraction and magnetic measurements. ErGe3 orders antiferromagnetically below TN = 7 K with a collinear antiferromagnetic moment arrangement with the magnetic space group C1121/m′ (P21/m′) (Sh1153). The Er magnetic moments are confined to the (a,b) plane making an angle of 15° with the a axis. The ordered magnetic moment value at 1.5 K is 8.6 μB per Er atom. Field-dependent magnetic measurements at 4.2 K show that ErGe3 is metamagnetic, and transforms to a ferromagnetic state with the same ordered magnetic moment value in fields larger than about 2 T.