Outstanding magnetic properties are highly desired for two-dimensional ultrathin semiconductor nanosheets for their potential applications in nano-electronics and spintronics. Here, ultrathin MoSe₂ nanoflakes with plenty of edges were prepared via an efficient chemical vapor deposition method. The magnetic measurement results indicate that the sample exhibits strong ferromagnetic behaviour with a saturation magnetization of 1.4 emu g⁻¹ at room temperature, where the ferromagnetism persists up to 700 K, revealing the high Curie temperature of this material. Density functional theory spin-polarized calculations predict that strong ferromagnetic moments in MoSe₂ nanoflakes are attributed to the zigzag edges. Our findings also suggest that the MoSe₂ nanoflakes with a high density of edge spins could be used to fabricate spintronic devices, which are circuits utilizing the spin of the electron to process and store information.