When the size of electric conductors is confined to several angstroms, quantum effects can be used to gain novel electron transport properties. We have shown that thanks to quantum effects, spin-polarized currents can be generated at the single-molecule scale without magnetic components or magnetic fields. Specifically, we detect spin-polarized currents in paramagnetic molecular junctions based on silver electrodes bridged by a vanadocene molecule. Remarkably, in some cases the obtained spin currents approach the limit of ideal ballistic spin transport. Detailed comparison between conductance and shot noise measurements to transport calculations reveals a mechanism based on spin-dependent quantum interference that yields very efficient spin filtering. Our findings pave the way for nanoscale spin-transport manipulations base on quantum interference, with the advantages of low sensitivity to decoherence effects in this scale, and the freedom to use non-magnetic materials for faster, power-saving spin manipulations.