A colorimetric and fluorescent dual chemosensor for detecting Al3+, (E)-N′-((2-hydroxynaphthalen-1-yl)methylene)nicotinohydrazide (HNN), was developed by combining 2-hydroxy-1-naphthaldehyde with nicotinohydrazide. Addition of Al3+ to EtOH–H2O solution of HNN gives rise to a visible colour change from colourless to light blue and a significantly enhanced fluorescence. Other metal ions including Na+, K+, Ca2+, Mg2+, Mn2+, Co2+, Ni2+, Cu2+, Cr3+, Ag+, Pb2+, Zn2+, Cd2+, Hg2+, Fe2+, and Fe3+ have no or little effect on the specific response of HNN to Al3+. The binding mode of HNN–Al3+ complex was further clarified by electrospray ionisation mass spectrometry and 1H NMR titration experiments.