Trithiocarbonate end‐functionalized poly(N‐acryloylglycinamide) (PNAGA), synthesized via reversible addition–fragmentation transfer (RAFT) polymerization of different molecular weights, is grafted onto gold nanoparticles (AuNPs) by ligand exchange in phosphate‐buffered saline. The PNAGA‐grafted AuNPs display upper critical solution temperature (UCST)‐type transitions: stable colloidal distribution and aggregation above and below the cloud points, respectively. Cloud points of PNAGA are not affected by the grafting procedure, and PNAGA@AuNPs show similar cloud points as that of the trithiocarbonate end‐functionalized free‐PNAGAs used for grafting. The UCST‐type phase transition is reversible for many cycles. The reversible control of the phase transition is proved by turbidity measurements and UV–vis spectroscopy, as well as by transmission electron microscopy (TEM). image