${\mathrm{LaCuS}}_2$ is a Cu-based rare-earth chalcogenide semiconductor whose structural robustness and pressure-dependent electronic evolution remain largely unexplored. Here, we investigate its structural, optical, and transport properties under compression by combining high-pressure experiments with first-principles calculations. ${\mathrm{LaCuS}}_2$ retains its monoclinic $P{2}_1/c$ structure up to 70.1 GPa without the emergence of a phase transition. Nevertheless, compression induces subtle local structural reorganization, including Cu-site disordering-to-ordering and an anisotropic lattice anomaly near 5.0 GPa associated with flattening of the corrugated corner-sharing ${\mathrm{Cu}}_2{\mathrm{S}}_6$ layers. Concurrently, the optical band-gap decreases continuously from 2.39 to 0.93 eV upon compression to 48.6 GPa, accompanied by distinct piezochromism and a progressive redshift of the photoluminescence emission. First-principles calculations reveal that the band-gap narrowing originates from the continuous evolution of the Cu $3d\text{−}\mathrm{S} 3p$ derived valence states and La $5d$ derived conduction states under compression. Electrical transport measurements show that ${\mathrm{LaCuS}}_2$ remains semiconducting up to 64.1 GPa, while photocurrent retains the same polarity throughout the investigated pressure range. These results establish ${\mathrm{LaCuS}}_2$ as an unusual pressure-robust semiconductor in which substantial band-edge engineering is achieved through continuous local structural evolution without a phase transformation, providing a model system for exploring pressure-induced electronic tuning in Cu-based rare-earth chalcogenides.

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• Band gap
• Crystal structure
• Density of states
• Electrical properties
• High-pressure studies
• Pressure effects
• Pressure techniques