The porphyrin marcocycle can stabilize a set of magnetic metal ions, thus introducing localized net spins near the center. However, it remains illusive but most desirable to introduce delocalized spins in porphyrins with wide implications, for example, for building correlated quantum spins. Here, we demonstrate that metal-free porphyrins host delocalized π-electron magnetism as revealed by scanning probe microscopy and different level of theory calculations. Our results demonstrate that engineering of π-electron topologies introduces spin polarized singlet state and delocalized net spins in metal-free porphyrins. In addition, the π-electron magnetism can be switched on/off via STM manipulation by tunning the interfacial charge transfer. Our results provide an effective way to precisely control the π-electron magnetism in metal-free porphyrins, which can be further extended to design new magnetic functionalities of porphyrin-based architectures.