Among the planets, Mercury has the least mass. The mass of Mercury is 3.302 x 10^23 kg. In comparison, our Earth has a mass of 5.9736 x 10^24 kg. On the other end of the scale, Jupiter is has the greatest mass, at 1.899 x 10^27 kg.
It is interesting to note that Pluto has a lesser mass than Mercury, having a mass of about 1.47 x 10^22 kg; however, Pluto is no longer designated as a planet and is instead a dwarf planet.
It is also important to note that even though Mercury is the lightest planet, it is certainly not the one with the lowest density (ratio of mass to volume). In fact, the density of Mercury (5427 kg/m^3) is only second to that of planet Earth (5514 kg/m^3). In comparison, Jupiter has a density of only 1326 kg/m^3.