Modern magnetic-memory technology requires all-electric control of perpendicular magnetization with low energy consumption. While spin-orbit torque (SOT) in heavy metal/ferromagnet (HM/FM) heterostructures(1-5) holds promise for applications in magnetic random access memory, until today, it has been limited to the in-plane direction. Such in-plane torque can switch perpendicular magnetization only deterministically with the help of additional symmetry breaking, for example, through the application of an external magnetic field(2,4), an interlayer/exchange coupling(6-9) or an asymmetric design(10-14). Instead, an out-of-plane SOT15 could directly switch perpendicular magnetization. Here we observe an out-of-plane SOT in an HM/FM bilayer of L1(1)-ordered CuPt/CoPt and demonstrate field-free switching of the perpendicular magnetization of the CoPt layer. The low-symmetry point group (3m1) at the CuPt/CoPt interface gives rise to this spin torque, hereinafter referred to as 3m torque, which strongly depends on the relative orientation of the current flow and the crystal symmetry. We observe a three-fold angular dependence in both the field-free switching and the current-induced out-of-plane effective field. Because of the intrinsic nature of the 3m torque, the field-free switching in CuPt/CoPt shows good endurance in cycling experiments. Experiments involving a wide variety of SOT bilayers with low-symmetry point groups(16,17) at the interface may reveal further unconventional spin torques in the future.