Polymeric probes with dithioacetal units in the side chains were designed and synthesized for the selective and efficient colorimetric sensing of Hg2+ ions in aqueous solutions. These polymeric sensors were prepared by a reaction between aldehyde groups of the side chain in the polymer and ethanethiol or 3-mercaptopropionic acid using BF3 as a Lewis acid. In aqueous solution, they exhibited a 30-40 nm red-shift in their absorption maxima upon the addition of Hg2+ ions, accompanied by a change in the color of the solution, from pale yellow to dark red. These results clearly demonstrated that the sensitive signaling behaviors originated from the Hg2+-promoted deprotection reaction of dithioacetal groups to form aldehyde functionalities. The sensors have excellent selectivity toward Hg2+ ions over other alkali and transition metal ions. The detection time for Hg2+ ion was finely tuned by a change in the pH of the solution. In particular, it took less than 1 min to complete Hg2+ ion detection at low pH. Given with fast and pH-tunable Hg2+ ion detection abilities, these polymeric probes are expected to offer unique potential platforms for integrating stimuli-responsive water-soluble polymers with tunable sensing behaviors.