We report long-range-ordered antiferromagnetism concomitant with local iron displacements in the spin-ladder compound BaFe${}_2$Se${}_3$. Short-range magnetic correlations, present at room temperature, develop into long-range antiferromagnetic order below $T_N$ $=$ 256 K, with no superconductivity down to 1.8 K. Built of ferromagnetic Fe${}_4$ plaquettes, the magnetic ground state correlates with local displacements of the Fe atoms. These iron displacements imply significant magnetoelastic coupling in Fe$X_4$-based materials, ingredients hypothesized to be important in the emergence of superconductivity. This result also suggests that knowledge of these local displacements is essential for properly understanding the electronic structure of these systems. As with the copper oxide superconductors two decades ago, our results highlight the importance of reduced dimensionality spin-ladder compounds in the study of the coupling of spin, charge, and atom positions in superconducting materials.

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