Uniaxial stress is a promising method to tune magnetic frustration, allowing its effects to be studied in a precise way. In this work, uniaxial stress is applied to the triangular-lattice antiferromagnet PdCrO (Formula presented) (Formula presented). The Cr–Cr magnetic interaction is very sensitive to interatomic separation, so laboratory-achievable stress can induce substantial changes in magnetic structure. Results from three types of measurement are presented: x-ray diffraction, the stress–strain relationship, and neutron diffraction. The combined data show that the elastic moduli of PdCrO (Formula presented) (Formula presented) are strongly affected by stress-induced changes in magnetic structure. A new, first-order stress-induced magnetic transition is observed, at which the lattice constant shrinks by 0.21%. The lattice stiffens dramatically across this transition: the Young’s modulus increases by (Formula presented) (Formula presented) GPa, and the Poisson ratio falls from (Formula presented) (Formula presented) to (Formula presented) (Formula presented). This stiffening indicates that the magnetic order ‘locks,’ that is, becomes insensitive to lattice strain. This locking might occur because the new stress-induced magnetic order nests the Fermi surface of the Pd sheets. Other frustrated magnets, including candidate spin liquids, may show similarly strong coupling between magnetic and elastic degrees of freedom.