A new metal organic framework [Co(OBA)-(DATz)(0.5)(H2O)] {OBA = 4,4'-oxybis(benzoic acid) and DATZ = 3,5-diamino-1,2,4-triazole}, 1, was synthesized by hydrothermal reaction. Single-crystal X-ray data of 1 confirmed two-dimensional rhombus grid network topology with a free nitrogen site of triazole ring and two amine groups of each DATZ. Photoluminescence study of 1 in aqueous medium shows blue emission at 407 nm upon excitation at 283 run. This emissive property was used for the sensing of Al3+ ions in aqueous medium through very high luminescence turn-on (6.3-fold) along with the blue shifting (similar to 24 nm) of the emission peak. However, luminescence studies in the presence of other common metal ions such as Mg2+, Zn2+, Ni2+, Co2+, Mn2+, K+, Na2+, Ca2+, Cd2+, Hg2+, Cu2+, Fe2+, Fe3+, and Cr3+ in aqueous medium shows luminescence quenching in varying extent. Interestingly, the luminescence turn-on-based selectivity of Al3+ ions in aqueous medium was achieved even in the presence of the highest quenchable metal ion, Fe3+. Furthermore, very high sensitivity was observed in the case of Al3+ ions with a limit of detection of Al3+ of 57.5 ppb, which is significantly lower than the higher limit of U.S. Environmental Protection Agency recommendation of Al3+ ion for drinking water (200 ppb).