A simple, efficient, and ecofriendly method has been developed for the exclusive synthesis of iron oxide nanoparticles using an aqueous extract of Passiflora tripartita var. mollissima fruit. Transmission electron microscopy (TEM) and Dynamic Light Scattering (DLS) analysis show that the average particle size of spherical iron oxide nanoparticles is 22.3 ± 3 nm. The synthesized nanocatalyst is highly active for the synthesis of biologically significant 2-arylbenzimidazoles. The reaction was carried out under mild condition with excellent yields. The catalyst is easily separated by magnet and recyclable without significant loss of activity.