<p>Strain in crystalline materials refers to the deviation of lattice sites from their ideal equilibrium positions. In crystalline semiconductors, the reliable management of strain is vital for simultaneously achieving high electronic quality and structural robustness. For metal halide perovskites, the complexity of material composition and crystallization dynamics introduces substantial challenges in strain engineering, hampering the long-term stability of perovskite devices. In fact, approaches for effective strain control in perovskite materials are yet to be explored. Here we engineer the strain distribution in the lead-free, tin-based perovskite CsSnI₃. Through a combination of simultaneous and sequential thermal evaporation of the perovskite precursors, we achieve an ideal condition in which the strain at the top and bottom interfaces of the perovskite is minimized, from −0.26% to −0.02%. Strain engineering enables ultra-stable lead-free perovskite light-emitting diodes with experimentally measured operational lifetimes of 1,250 h and 3,350 h under intense currents of 100 mA cm⁻² and 25 mA cm⁻², respectively. The device lifespan (at 100 mA cm⁻²) outperforms the most stable lead-based perovskite light-emitting diodes by ~5 times and lead-free perovskite light-emitting diodes by ~31 times. Our work contributes to resolving the long-standing trade-off between eco-friendliness and stability in perovskite devices.</p>