We report the synthesis of thermophilic poly(N-acryloylglycinamide) (PNAGA) microgels that swell in water upon heating and their use as nanocatalyst hosts. The microgels are prepared by aqueous precipitation polymerization below the UCST phase transition temperature of PNAGA. The diameters of the PNAGA microgels are around 60 nm in cold water and they show reversible swelling and shrinking upon change of temperature. Silver nanoparticle (AgNP) loaded microgels were prepared by reduction of AgNO₃ and their catalytic activity in 4-nitrophenol reduction was tested under different conditions. The thermophilic behaviour of the PNAGA microgels was retained after nanocatalyst synthesis and the catalytic activity in 4-nitrophenol reduction by AgNPs increased especially at temperatures above 30 °C. Furthermore, it was shown that the catalytic activity of the thermosensitive AgNP–PNAGA microgels could be switched on and off by changing the temperature.