Liquid Hg undergoes the metal–nonmetal (M–NM) transition when it is expanded from 13.6 g cm−3 at ambient conditions to 9 g cm−3 at high temperature and high pressure. To investigate collective and single particle motions in expanded fluid Hg, we have made inelastic X-ray scattering experiments and obtained the dynamic structure factor, S(Q,ω), of fluid Hg. We analyzed S(Q,ω) within the framework of generalized hydrodynamics and found that the excitation energies of collective modes disperse three times as fast as the hydrodynamic sound velocity in the M–NM transition region at 9 g cm−3. The results indicate the existence of fast sound in expanded fluid Hg accompanying the metal–non-metal transition and strongly hint that fluctuations intrinsic to the M–NM transition are induced on atomic length scale and sub-picosecond time scale.