A completely water soluble azobenzene chemosensor 1 for selective detection of Hg²⁺ was synthesized. Taking advantage of the absorption changes corresponding to the transition moments polarized along the short axis of an azobenzene, 1 showed characteristic UV-Vis signal changes in the band around 240 nm for Hg²⁺ in wide pH ranges, which also showed good tolerance to various metal ions and photoirradiation. Upon addition of Hg²⁺ into the solution of 1, a favored formation of trans-1 was observed, which is attributed to an intramolecular coordination of the PEG chain and Nβ to Hg²⁺ confirmed by a control experiment test.