<p>Altermagnetism is a magnetic phase that combines zero net magnetization with time-reversal-symmetry breaking and momentum-dependent spin splitting, but it has so far been confined to fermionic systems. Here we report a photonic platform capturing the symmetry features of altermagnetism. Using a magnetophotonic crystal with staggered magnetic bias and controlled structural variation, we observe momentum-dependent polarization splitting, spin–momentum locking and vanishing net magnetization in the photonic band structure. By solving Maxwell’s equations, we show that the momentum-dependent splitting is symmetry governed, rather than a consequence of the momentum-independent gyrotropic effect. These results demonstrate a photonic analogue of altermagnetic behaviour and provide a route towards spin-functional photonic devices without net magnetization.</p>