We report on the use of photoinitiated reversible addition-fragmentation chain transfer (RAFT) polymerization for the facile fabrication of cross-linked nanoporous polymer films with three-dimensionally (3D) continuous pore structure. The photoinitiated polymerization of isobornyl acrylate (IBA) in the presence of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (CTA) and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator proceeded in a controlled manner, yet more rapidly compared to thermally initiated polymerization. When polylactide-macroCTA (PLA-CTA) was used, PLA-b-PIBA with high molar mass was obtained after several minutes of irradiation at room temperature. We confirmed that microphase separation occurs in the PLA-b-PIBA and that nanoporous PIBA can be derived from the PLA-b-PIBA precursor by selective PLA etching. To fabricate the cross-linked nanoporous polymer, IBA was copolymerized with ethylene glycol diacrylate (EGDA) in the presence of PLA-CTA to produce a cross-linked block polymer precursor consisting of bicontinuous PLA and P(IBA-co-EGDA) microdomains, via polymerization-induced microphase separation. We demonstrated that nanoporous P(IBA-co-EGDA) monoliths and films with 3D continuous pores can be readily obtained via this approach.