Swells are waves that undergo minimal attenuation and can travel thousands of kilometers, serving as a non-local factor that greatly influences air–sea interactions and maritime activities. Although various empirical formulas for swell decay have been proposed, the key parameter governing this process remains largely uncertain. This study combines global ERA5 wave reanalysis data with WAVEWATCH III simulations to examine swell decay rates. Results indicate that wave steepness plays a fundamental role in the decay process. An analytical model that incorporates wave steepness predicts a reciprocal scaling of significant wave height for swells, which differs from traditional exponential decay and aligns more closely with simulation results across various conditions. Our findings offer a physically grounded method for enhancing swell-dissipation parameterization and reducing wave-modeling ambiguity.