Recent explorations of the state-dependence of Earth’s equilibrium climate sensitivity (ECS) have revealed a pronounced peak in ECS at a surface temperature of approximately 310 K. This ECS peak has been observed in models spanning the model hierarchy, suggesting a robust physical source. Here we propose an explanation for this ECS peak using a novel spectrally-resolved decomposition of clear-sky longwave feedbacks. We show that the interplay between spectral feedbacks in H2O- and CO2-dominated portions of the longwave spectrum, along with moist-adiabatic amplification of upper-tropospheric warming, conspire to produce a minimum in the feedback parameter, and a corresponding peak in ECS, at a surface temperature of 310 K. Mechanism denial tests highlight three key ingredients for the ECS peak: 1) H2O continuum absorption to quickly close spectral windows at high surface temperature; 2) moist-adiabatic tropospheric temperatures to enhance upper-tropospheric warming; and 3) energetically-consistent increases of CO2 with surface temperature.