The discovery of n-type ferromagnetic semiconductors (n-FMSs) exhibiting high electrical conductivity and Curie temperature (T-c) above 300 K would dramatically improve semiconductor spintronics and pave the way for the fabrication of spin-based semiconducting devices. However, the realization of high-T-c n-FMSs and p-FMSs in conventional high-symmetry semiconductors has proven extremely difficult due to the strongly coupled and interacting magnetic and semiconducting sublattices. Here we show that decoupling the two functional sublattices in the low-symmetry semiconductor FeBi2Se4 enables unprecedented coexistence of high n-type electrical conduction and ferromagnetism with T-c approximate to 450 K. The structure of FeBi2Se4 consists of well-ordered magnetic sublattices built of [FenSe4n+2](8) single-chain edge-sharing octahedra, coherently embedded within the three-dimensional Bi-rich semiconducting framework. Magnetotransport data reveal a negative magnetoresistance, indicating spin-polarization of itinerant conducting electrons. These findings demonstrate that decoupling magnetic and semiconducting sublattices allows access to high-T-c n- and p-FMSs as well as helps unveil the mechanism of carrier-mediated ferromagnetism in spintronic materials.