Bulk MoS₂, a prototypical transition metal chalcogenide material, is an indirect band gap semiconductor with negligible photoluminescence. In this study, we have developed, for the first time, a simple and low-cost synthetic strategy to prepare boron- and nitrogen-doped MoS₂ (B,N-MoS₂) nanosheets. Through boron and nitrogen doping, the band gap of MoS₂ increases from 1.20 eV to 1.61 eV, and the obtained B,N-MoS₂ nanosheets exhibit an enhanced fluorescence. The B,N-MoS₂ nanosheets can be used as a green and facile sensing platform for label-free detection of Hg²⁺ because of their high sensitivity and selectivity toward Hg²⁺. In addition, detection can be easily accomplished through one-step rapid (within 2 min) operation, with a limit as low as 1 nM. This study demonstrates that the introduction of boron and nitrogen elements into ultrathin MoS₂ nanosheets for enhanced fluorescence properties is feasible through a facile and general preparation strategy and may also offer a unique idea as a potential way to design more efficient MoS₂-based sensors and fluorescent materials.