The generation and transport of spin currents are central to spintronics, a technologically relevant field in which information is carried and manipulated using the electron spin rather than solely the electrical charge. Conventionally, electrical spin injection and the separation of charge and spin currents rely either on ferromagnets^{1, 2, 3, 4, 5, 6, 7, 8, 9–10}, in which spin polarization originates from a net magnetization, or on relativistic effects such as the spin Hall effect^{11, 12, 13, 14, 15–16}. Here we demonstrate spin injection from Mn₅Si₃, a compensated magnet with vanishing net magnetization. Using lateral spin-valve devices, we electrically inject spins from Mn₅Si₃ into a non-magnetic channel and detect their propagation at a spatially separated ferromagnetic electrode. Switching between time-reversed magnetic states of Mn₅Si₃ reverses the detected spin signal, demonstrating that the spin injection breaks time-reversal symmetry. This behaviour is observed for two distinct electrical-biasing geometries, providing a characteristic signature that differs from conventional ferromagnetic and spin-Hall injection. Symmetry analysis, first-principles calculations and spin-transport modelling show that our observations are consistent with the altermagnetic order of Mn₅Si₃, in which spin-polarized currents can arise despite the absence of net magnetization^{17, 18, 19, 20–21}. The calculations yield spin polarizations of 6.5–10%, comparable to those of conventional ferromagnets, whereas the spin-transport modelling of the experiments indicates spin diffusion lengths of 50–130 nm. Our results provide experimental evidence that a compensated magnet can generate spin currents controlled by its magnetic order and demonstrate that the altermagnetic properties of Mn₅Si₃ enable spintronic functionality without net magnetization.