Display omitted •A sensitive fluorescent chemosensor for iodide, based-on a cationic porphyrin, 1 was synthesized and studied in pure water.•1 selectively detects I− ions in the nanomolar concentration range with good selectivity over common inorganic anions.•Iodide forms an very stable ion-pair complex and quenches efficiently its red fluorescence.•Iodide sensing was studied by spectroscopic experiments, fluorescence-lifetimes, crystal structure and DFT calculations. The new tetracationic and highly luminescent triflate salt of 5,10,15,20-tetra(4-benzylpyridinium)-21H,23H-porphyrin, 1.OTf was designed and studied as a chemosensor for iodide in pure water. Under optimal conditions, the addition of inorganic anions and acetate as sodium salts to 1.OTf quench its red fluorescence with a very pronounced affinity toward iodide (K1.I−  = 1.9(±0.08) x 104 M-1) accompanied by complete quenching of its emission (KSV = 1.1(±0.1) x 105) in a micromolar concentration range (0-50 μM). A rapid and direct analytical response of 1.OTf by addition of I- ions was observed in a wide range of pH (5.0–8.0) with a detection limit of 180 nmol L-1 at neutral pH and good selectivity over other biological and more basic anions such as pyrophosphate, chloride, fluoride and phosphate. The optical change by addition of iodide is attributed to a strong ion-pair binding with simultaneous formation of efficient hydrogen bonding C–H … I- interactions, these contacts are supported by 1H NMR experiments. The efficient binding and fluorescence detection is attributed to a very high positive charge of monomeric specie of 1 in combination with strong acidification of C–H donors by quaternization of pendant pyridyl groups. On the other hand, addition of bromide, common interfering anion, shows a modest spectral change but lower than those observed for iodide. On basis of multiple spectroscopic titration experiments (1H NMR, UV–vis, fluorescence), Job´s plot, crystal structure of the bromide salt of 1 and fluorescence lifetime measurements, a photoinduced electron transfer quenching mechanism with simultaneous sensor-iodide complexation both in the excited and the ground-state is proposed. From theoretical DFT studies, the binding mode of this chemosensor with iodide is markedly ionic interaction, energy calculated in 402.8 kcal/mol. The proposed method is direct, fast, accurate and of low cost for the determination of iodide ions in pure water.