Two-dimensional ferromagnetic ultrathin nanosheets hold great promise for next generation electronics and spintronics. Here, intrinsic ferromagnetism was achieved through a new effective strategy by fluorine adsorption on MoS2 nanosheets, where the fluorinated MoS2 nanosheets exhibit stable ferromagnetic hysteresis at room temperature with saturation magnetization of 0.06 emu g(-1) and magnetoresistance of 4.1%. The observed ferromagnetism can be tuned by changing the concentration of the adatom. On the basis of first-principle calculations, it is shown that not only fluorine absorbed MoS2 monolayer favours spontaneous spin polarization and local moment formation, but also that the spin moments can exhibit long range magnetic ordering. This work paves a new pathway to engineer the magnetic properties of the two-dimensional nano-materials.