The new polymeric photosensitizer poly[(N‐isopropylacrylamide)‐co‐(vinylbenzyl chloride/Rose Bengal)] for singlet oxygen production was synthesized and studied. The polymer is soluble in polar solvents, such as methanol and water. It displays a lower critical solution temperature in aqueous solutions at 31.7 ± 0.3°C. The quantum yield of singlet oxygen formation by the Rose Bengal‐polymer bound chromophore in methanol solution is close to that determined for free Rose Bengal. In an aqueous solution of the polymer, the efficiency of singlet oxygen formation is considerably lower. This phenomenon results mainly from the aggregation of the Rose Bengal chromophores.