Thermoresponsive membranes with nanoscale pores were successfully fabricated. The membranes were composed of a poly(ethylene glycol methyl ether methacrylate)-b-polystyrene-b-poly(ethylene glycol methyl ether methacrylate) (PMENMA-b-PS-b-PMENMA) mesoporous size selective layer attached to a polyvinylidene fluoride (PVDF) macroporous supporting layer. Mesopores were introduced into the hydrophilic PMENMA domains by controlled selective swelling with methanol and supercritical CO2. Because selective swelling with methanol occurs only in the hydrophilic PMENMA block domains, the mesopore interiors are covered with PMENMA brushes. PMENMA has a lower critical solution temperature (LCSTs) in water that depends on the number of ethylene glycol units in the oligo(ethylene glycol) side chain (N), thus providing a means of tuning the thermoresponsivity. Water permeability experiments showed that a higher flux of water passes through the membranes at temperatures greater than the LCST, suggesting that the pore sizes are temperature controllable. Also, with increasing N, the response temperature increased. The thermo-dependent size selectivity of the membranes was also investigated, and the results show that the membranes have very small pore sizes, about 5 nm, and a strong size-discriminating property. The temperature dependent changes of water flux and particles permeation all demonstrate the thermoresponsivity of the membranes; however, the open-close transition of the pores was not sudden at the LCSTs but instead occurred gradually over a broad temperature range.