A highly fluorescent triazine-bridged polymer, poly[(diphenylamino-s-triazine)-co-(2-methoxy-5-propyloxysulfonate-1,4-phenylene vinylene)] (DTMSPV), is synthesized from Wittig polycondensation of a triazine monomer with a water-soluble p-phenylene vinylene monomer. The fluorescent amphiphilic polymer in aqueous solution self-assembled into nanoassemblies of micelle-like nanostructure (MS) and π stacking nanostructure (πS), which have average sizes of 93 to 270 nm, depending on the concentration of DTMSPV. The micelle-like nanostructure of DTMSPV (MS) shows blue emission at 457 and 488 nm with a high emission quantum yield (ΦE) of 31% in aqueous solution. On the other hand, the ΦE of π stacking structures (πS), formed in a highly concentrated solution, is lower than the MS. The MS exhibits fluorescence quenching as well as color change from blue to green/yellow, depending on the kinds of metal ions. The metal ion sensitivity is larger in the order of the main group ions (Na+, K+) < dicationic transition metal ions (Zn2+, Cd2+, Pb2+, Cu2+, Pd2+) < trivalent transition metal ions (Fe3+, Ru3+), with an exception of Al3+. In particular, the fluorescence of MS is dramatically quenched with color change to yellow in response to Al3+ concentrations. The selectivity and sensitivity of MS to Al3+ are unusually high even in the presence of competitive metal ions, which can be attributed to the specific interaction of triazine units with Al3+.