Boehmite nanoparticles are aluminum oxide hydroxide (γ-AlOOH) particles, which were prepared by a simple and inexpensive procedure and immobilized further with nickel-S-methylisothiourea complexes (Ni-SMTU@boehmite), and characterized by FT-IR spectroscopy, TGA, XRD, ICP-OES, EDS, SEM and TEM techniques. Finally, we investigated the catalytic activity of Ni-SMTU@boehmite as a new, heterogeneous, stable and highly reusable nanocatalyst for the synthesis of 5-substituted 1H-tetrazole derivatives in PEG-400 as a green solvent. This catalyst was recovered by simple filtration and reused several times without significant loss of its catalytic efficiency or nickel leaching. The heterogeneity of this catalyst has been studied using hot filtration and ICP-OES technique. © 2016 The Royal Society of Chemistry.