Abstract Unusual two-dimensional (2D) and three-dimensional (3D) magnetic ordering phenomena were observed in the insulating, transparent erbium trihalides ErX 3 (X=Cl, Br, I) in the milli-Kelvin temperature range. The layer-type crystal structures of ErCl 3 (AlCl 3-type structure) and ErBr 3/ErI 3 (BiI 3-type structure) are closely related. All three compounds show a new type of two-sublattice 120° antiferromagnetic order in the erbium honeycomb layers with an infinite rotational degeneracy. The magnetic structures were determined by powder and single-crystal neutron diffraction. ErCl 3 shows short-range 2D magnetic order above 350 mK to several Kelvin and a transition to 3D magnetic order at 350 mK with a k -vector of ( 2 3 , 0, − 1 12 ). ErBr 3 and ErI 3 display short-range 2D magnetic order from 400 mK to several Kelvin, long-range 2D order between 280 and 400 mK, and short-range 3D order below 280 mK with a correlation length of 15 Å along the c-axis and k=( 1 3 , 1 3 , 0) . The differences between ErCl 3 and ErBr 3/ErI 3 are due to different geometrical layer stackings. For ErBr 3/ErI 3 a threefold ambiguity in the magnetic layer stacking causes the disorder along the c-axis. The ordered magnetic moments at saturation increase from 3.3 to 4.7 and 5.5 μ B/Er 3+ for ErCl 3, ErBr 3 and ErI 3, respectively.