||For the detection of mercury ion, many kinds of chemosensors and biosensors have been developed using organic chromophores, semiconductor quantum dots, and conjugated polymers. The development of efficient sensors utilizing conjugated polymers has gained much attention. Especially, water-soluble conjugated polymers show exceptional fluorescence quenching efficiencies in the presence of oppositely charged acceptors and biological materials. With biological materials, thymine have a unique feature, which it can be coordinated with mercury to form mercury-thymine-mercury complex. For this reason we synthesized novel water-soluble conjugated polymer containing thymine side chains and bisthienylbenzothiadiazole (TBT) units. The polymers exhibited a blue-to-red fluorescence change upon the addition of mercury, due to exciton migration.