Altermagnets feature antiparallel spin sublattices with $d$-, $g$-, or $i$-wave spin order, yielding nonrelativistic spin splitting without net magnetization. In this Letter, we embed a two-dimensional $d$-wave altermagnet in a driven terahertz cavity. Because the intrinsic altermagnetic texture renders the electronic sublattices locally inequivalent, coherent cavity driving then couples to them asymmetrically, breaking the spin-sublattice transposing symmetries (such as [$C_2\parallel C_4$] or [$C_2\parallel M$]), generating a nonequilibrium sublattice population imbalance that produces a steady-state magnetization—a response absent in conventional antiferromagnets with the same lattice geometry. A mean-field Lindblad analysis reveals the dominance of quadratic over linear couplings. In the strong-coupling regime, distinct polariton signatures emerge in the steady state of induced magnetization. This Letter demonstrates cavity control of altermagnets for spintronic applications.

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• Altermagnetism
• Cavity quantum electrodynamics
• Light-induced magnetic effects