Altermagnets have recently attracted considerable interest due to their unique symmetry-governed spintronic properties. Here, we investigate phonon-induced magnon spin currents in a two-dimensional altermagnet. Starting from a microscopic theory of the coupled magnon–phonon system, we derive the nonequilibrium magnon distribution generated by selective phonon excitations. We show that the resulting spin currents exhibit a pronounced d-wave symmetry with respect to the phonon momentum. Moreover, the spin current along the altermagnetic directions can be completely reversed by tuning the phonon frequency. These findings establish altermagnets as promising platforms for realizing highly tunable, phonon-driven coherent terahertz magnon spin currents.