The preparation and characterization of a series of poly(methyl methacrylate)-block-poly(butyl acrylate)-block-poly(methyl methacrylate) ABA triblock copolymers and their clay nanocomposites via in situ ATRP is reported. The molecular weights and the chemical structures of these nanocomposites are characterized by means of GPC, FTIR, and NMR analyses. DSC, AFM, and SAXS analyses prove a nanophase-separated morphology in the block copolymer/clay nanocomposites. DMA analysis demonstrates an additional reinforcing effect of nanoclay in the block copolymer nanocomposites. AFM analysis indicates that the bulk polymer has a cylindrical morphology that is unaffected in the nanocomposite, as corroborated by SAXS analysis.