<p>The diurnal cycle of sea surface skin temperature (SST_skin) plays a critical role in the air-sea system by modulating oceanic and atmospheric processes on longer timescales. The European Centre for Medium-Range Weather Forecasts (ECMWF) forecasting system has taken the diurnal cycle of SST_skin into account by incorporating parameterization schemes for warm-layer and cool-skin effects, and provides hourly SST_skin outputs in ECMWF Reanalysis v5 (ERA5 reanalysis). This study evaluates the SST_skin diurnal amplitude (i.e., the diurnal temperature range) and phase (i.e., the timing of the daytime maximum) in ERA5 using observations from the Global Tropical Moored Buoy Array (GTMBA). Compared to GTMBA, the SST_skin diurnal amplitude in ERA5 is significantly underestimated, with a mean bias of −0.12 K and a percentage error of 36%. The amplitude error declines with increasing surface wind speed but grows with increasing surface shortwave radiation. The timing of the daytime SST_skin maximum in ERA5 is substantially advanced, with an average lead of 1.69 h, and the lead time increases with increasing wind speed/shortwave radiation. These results suggest that the warm-layer parameterization scheme incorporated in the ECMWF system likely underestimates the daytime warming from air-sea heat exchange and/or overestimates the cooling effect of oceanic turbulent mixing, providing insights for future improvements of this scheme.</p>