A thiosemicarbazone-based water-soluble polymeric probe was developed for the selective colorimetric detection of Cu(II) ions with pH-tunable sensitivity. The successful separation of Cu(II) ions from several alkali and transition metal cations by thermal precipitation was demonstrated. N,N-Dimethylacrylamide (DMA) and 3-vinylbenzaldehyde (VBA) were copolymerized by reversible addition-fragmentation chain transfer (RAFT) polymerization to produce p(DMA-co-VBA), herein P1. The aldehyde group of P1 was reacted with 4-phenylthiosemicarbazide to yield poly{DMA-co[N-phenyl-2-(3-vinylbenzylidene)hydrazinecarbothioamide]}, [p(DMA-co-PVHC)], herein P2. Upon the addition of Cu(II) ions to an aqueous solution of P2, the color of the solution turned from colorless to yellow due to the formation of a coordination complex between the Cu(II) ions and phenylthiosemicarbazone units of P2, herein P2 Cu(II). The availability of the electron-rich imino nitrogen in the thiosemicarbazone units allowed P2 to show excellent sensing behavior toward Cu(II) ions. P2 has remarkable pH-switchable sensing properties toward Cu(II) ions. Although efficient colorimetric sensing was observed at neutral or high pH, no appreciable color change appeared at low pH. Protonation of the imino nitrogen at low pH prevented the formation of coordination complexes. Cu(II) ions were separated successfully from various alkali and transition metal cations by thermal precipitation due to the thermoresponsive property of P2-Cu(II).