Abstract Understanding sub‐daily variability in phytoplankton proxies is essential for interpreting satellite estimates of ocean productivity in marginal seas. Using high‐frequency observations from the Geostationary Ocean Color Imager (GOCI), we assess daytime (08:00–15:00 local time) hourly surface chlorophyll‐a (Chla) variability across China’s marginal seas. Satellite‐derived Chla exhibits a reproducible three‐phase sub‐daily pattern, with a morning decrease, a mid‐day increase, and an afternoon stabilization or decline, showing clear seasonal and regional contrasts. Hourly Chla variability displays time‐of‐day–dependent associations with net surface solar radiation (NSSR), including positive correspondence under low to moderate irradiance, a mid‐day peak under intermediate light, and reduced or negative correspondence under high afternoon irradiance. Northern marginal seas show larger amplitudes than southern regions, particularly during spring and summer. These results demonstrate the value of geostationary observations for resolving robust sub‐daily chlorophyll patterns and underscore the importance of time‐of‐day and regional context when interpreting satellite‐derived Chla variability.