A “turn‐on” fluorescent sensing strategy was developed for ultrasensitive detection of heavy metal mercury ion (Hg²⁺) directly in water. It utilized an aqueous soluble aggregation‐induced emission (AIE)‐active probe, named 1,1,2,2‐tetrakis(4‐(1H‐tetrazol‐5‐yl)phenyl)ethene (TPE‐4TA), composing of a luminescent tetraphenylene core and multiple anionic tetrazolate spawns. The probe retained dark when molecularly dissolved in water. However, mercury ions can ligate with multiple tetrazolate groups to form infinite coordination polymer particles spontaneously, thus inducing a fluorogenic AIE response for in situ analysis. Moreover, by embedding this AIE sensing molecules in a hydrophilic polyvinyl alcohol substrate, the resulting hydrogel film allowed in situ detection of Hg²⁺ on a laser‐induced fluorescence analysis setup in aqueous environment. This strategy worked effectively in common aqueous environments with pH from 4 to 7.5 and showed high sensitivity (limit of detection down to 0.38 ppb/1.9 nM), good selectivity and a wide linearity range for quantification. This work may lead to a reliable and promising platform for on‐site environmental water analysis.