With the rapid development of modern nanotechnology, molecular self-assembly has become an important method to fabricate new functional devices, and to provide an arena for theoretical material designs. In this paper, we propose that freestanding two-dimensional organometallic porous sheets (PSs), which can be formed by molecular self-assembly on metal surfaces, are ideal low-dimensional magnetic materials with room-temperature ferromagnetism. Through comprehensive first-principles calculations, we show that the freestanding organometallic sheets, which are assembled by transition metals (TMs) (Mn and V) and benzene molecules, favor ferromagnetic coupling with strong exchange interactions. More importantly, we predict that the Curie-temperature of V-PS is close to room temperature using a simplified mean-field expression, compared to any organometallic sheets discovered previously. In terms of the recent progress in the molecular self-assembly approach, our results indicate great potential for building room-temperature magnetic organometallic sheets with small magnetic molecules.