In this study, we tested successfully a green method for the synthesis of gold nanoparticles using Piper longum fruit extract (PLFE) along with data on their in vitro antioxidant and catalytic activities. The formation of P. longum gold nanoparticles (PLAuNPs) was confirmed by UV-visible spectroscopy. The average size of the PLAuNPs was 56 nm as confirmed by the DLS particle size analyzer. The TEM-EDX revealed that PLAuNPs were spherical in shape and contained metallic gold. FTIR analysis indicated that the phenolic groups present in PLFE were involved in the reduction and capping of PLAuNPs. The thermal stability of PLAuNPs was analyzed by the TGA. PLFE and PLAuNPs exhibited moderate free radical quenching ability when analyzed by various in vitro antioxidant assays. The catalytic activity of PLAuNPs against four organic dyes namely methyl blue, methyl red, crystal violet and acridine orange was tested, which resulted in respective degradation of 65, 28, 39 and 34% after 28 h of mixing. Such green nanoparticles with potent catalytic properties might be useful in clearing the toxic dyes in industrial effluents. (C) 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.