We demonstrated a novel approach to produce robust bicontinuous nanoporous polymer membranes by ring-opening metathesis polymerization of a new norbornene-functional diblock copolymer and dicyclopentadiene followed by selective removal of a chemically etchable polylactide block. Polymerization-induced phase separation from the initial homogeneous tetrahydrofuran solution of all components led to a disordered but cocontinuous structure in the resulting cross-linked plastics. After polylactide removal, the nanoporous (ca. 20 nm pores) membranes exhibited excellent thermal stability and mechanical toughness, high specific surface areas, narrow pore size distributions, and good permeability.