A simple, cheap and ultrasensitive colorimetric Hg2+ sensing strategy has been developed in this paper. It was based on a special 'Hg2+-inhibited aggregation' mechanism, in which the pyridine-induced aggregation of unmodified gold nanoparticles (AuNPs) could be inhibited upon addition of Hg2+. Compared with the previous Hg2+-induced aggregation mechanism, the new design just employed a cheap and facile chemical reagent, pyridine, as an inducer of aggregation of AuNPs and elimination of the modifying or labeling step. The effects of pyridine concentration and size of AuNPs were investigated. The calibration curve showed that the extinction ratio value at 525 and 700 nm increased linearly over the Hg2+ concentration range of 0.15-3.00 µM with a detection limit of 55 nM. The specificity of this sensor is remarkably high over that of the other metal ions without adding any masking agent.