We performed inelastic neutron scattering experiments on a polycrystalline sample of the two-dimensional $d$-wave altermagnetic candidate ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Mn}}_2{\mathrm{Se}}_2$, which features a Lieb lattice geometry. The magnetic-excitation spectrum is well reproduced by linear spin-wave theory. The analysis yields a first-neighbor exchange interaction of $J_1=2.34\left(3\right)\text{meV}$ and two distinct second-neighbor altermagnetic exchange interactions of $J_2=0.56\left(2\right)\text{meV}$ and $J_2^{\prime }=0.41\left(4\right)\text{meV}$, establishing that the material is a two-dimensional $d$-wave altermagnet. These interaction values are consistent with first-principles calculations. This demonstration of a $d$-wave altermagnet that allows magnon spin currents underscores the promise of altermagnets for magnonics.

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• Altermagnetism
• Magnetism
• Inelastic neutron scattering