Hidden Staggered Superconducting Order in V2Se2O-Family Altermagnets

$\eta$ pairing describes a staggered superconducting condensate whose order parameter changes sign between neighboring unit cells while maintaining a uniform magnitude. Nearly four decades after its theoretical proposal by C. N. Yang, a material platform for stabilizing $\eta$ pairing has remained elusive. Here, we reveal that altermagnetism, despite being pair breaking for conventional BCS pairing, naturally favors $\eta$-like pairing in V${}_2$Se${}_2$O-family altermagnets. Their valley-contrasting spin structure gives rise to oppositely spin-polarized, symmetry-related valleys centered at $X=(\pi ,0)$ and $Y=(0,\pi )$, which form spin-singlet pairing partners separated by the commensurate momentum $\mathbf{Q}=(\pi ,\pi )$, thereby favoring an $\eta$-like condensate. Self-consistent calculations demonstrate that the resulting $\eta$-like state remains robust over broad variations of chemical potential, hopping parameters, and Fermi-surface topology. To detect this hidden staggered order, we propose a phase-sensitive Josephson measurement with a uniform superconductor, where destructive interference suppresses single-Cooper-pair tunneling while preserving two-Cooper-pair processes. This constitutes a charge-$4e$-like Josephson response despite a bulk condensate composed of charge-$2e$ Cooper pairs, providing a direct phase-sensitive signature of the otherwise elusive $\eta$-like condensate in V${}_2$Se${}_2$O-family altermagnets.