<p>Marine heatwaves are typically monitored using “foundation” sea surface temperature to ensure a stable climate reference, but this practice neglects the short-lived diurnal extremes occurring within the upper ~10 m of the ocean, where near-surface organisms are exposed and air-sea exchange processes are modulated. Using satellite observations, atmospheric reanalysis, and ocean model simulations, we show that during marine heatwave conditions, the diurnal cycle is systematically amplified, with enhanced daytime warming and delayed diurnal phases. Globally, daytime subskin peak is intensified by 20–30%, elevating event-mean maximum intensity by 0.5–1.0 °C and adding tens of degree-days to cumulative exposure. This widespread amplification arises from a thermodynamic feedback in which weak winds and enhanced shortwave radiation during marine heatwaves shoal and stabilize the mixed layer, allowing near-surface heat to accumulate and persist into evening hours. In low-latitude oceans, up to 40% of marine heatwave days exceed the foundation-defined event peak at the subskin level, exposing ecosystems to unaccounted thermal stress. We suggest well-resolved diurnal, subskin-based diagnostics need to be integrated into future marine heatwave monitoring and prediction frameworks.</p>