Ag nanoclusters functioned by hyperbranched polyethyleneimine have been developed as a new fluorescent and colorimetric platform for sensitive and selective recognition of halide ions (e.g., Cl–, Br–, and I–). The recognition mechanism is based on the unique reactions between halide ions and the silver atoms. In particular, halide-induced oxidative etching and aggregation can produce a strong fluorescence quenching of Ag nanoclusters. This sensing system exhibits a remarkably high selectivity toward halide ions over most of anions and cations and shows good linear ranges and lower detection limits: the linear ranges are 0.5–80 μM for Cl–, 0.1–14 μM for Br–, and 0.05–6 μM for I–, respectively; the limits of detection for Cl–, Br–, and I–, at a signal-to-noise ratio of 3, are estimated to be 200, 65, and 40 nM, respectively. Specifically, Br– and I– could be recognized selectively in the coexistence with Cl– under the condition of higher ionic strength, which is a significant advantage in the detection of Br– and I– in real samples. In addition, the recognition of halide could be performed by the colorimetric method, which is also attractive and promising because of its simplicity, rapidity, reliability, and low cost. Furthermore, this sensing system has been applied successfully to the detection of Cl– in real water samples.