We present size-based separation of nanoparticles using deterministic lateral displacement (DLD) arrays in poly(dimethylsiloxane) (PDMS). Unlike conventional dry-etched silicon DLD devices for nanoparticle separation, our proposed devices are fabricated from PDMS by standard soft lithography. Using a DLD device having a critical diameter D_c of 0.7 µm, we could separate fluorescent polystyrene particles of diameters 1 and 0.5 µm. In addition, we could demonstrate solution exchange for 0.5-µm beads by using a DLD device having a smaller D_c of 0.3 µm.