The perovskite YRuO 3 containing low-spin d 5 Ru 3+ has a Pnma orthorhombic structure with elongated RuO 6 octahedra evidencing orbital order and is insulating to low temperatures with a band gap of 70 meV. Canted antiferromagnetic Ru 3+ spin order is observed below T C = 97 K, and magnetization plateaus that emerge below 62 K reveal a quantum weak ferromagnetic state where 1/8 of the spins are reversed, reflecting extreme singleion anisotropy resulting from the strong spin-orbit coupling of the d 5 Ru 3+ ion. Dynamic effects of the reversed spins give rise to an unusual negative AC susceptibility response, and a partial freezing of this motion occurs at 27 K.