The analysis of long-term tide gauge data collected in the Northeast Atlantic Ocean reveals that the seasonal cycle of mean sea level (hereafter MSL) exhibits amplitudes of up to 0.4 m. The position of MSL is of fundamental importance for many issues such as storm-induced flooding or the morphodynamics of shallow inlets, yet the underlying mechanisms are not fully understood. We characterize the seasonal cycle based on field observations complemented with a numerical hindcast. We analyzed long-term series (2000-2010) of in situ tide gauge data along the coasts of Portugal, Spain and France. The combined analysis of field observation and model results revealed that atmospheric pressure, wind and steric effect are the main contributors to the seasonal cycle of MSL along North East Atlantic Ocean coastlines. We find a coherent signal over the region: the cycle peaks around November and has its minimum in February. Monthly mean sea level rises slowly and falls quickly. The different features between north and south mirror the different forcing mechanisms acting in each area. To the north, the seasonal cycle of MSL is more irregular and controlled by atmospheric forcing because this region is on the track of low-pressure storms, especially during winter. To the south, the steric effect plays an important role mostly due to the persistence of high pressure and a narrow continental shelf. Our results suggest that for a given storm, the water level and subsequent flooding damage will be higher if it occurs at the end of the autumn than at the end of the winter, which suggest that the seasonal cycle of MSL should be represented in flooding modeling systems.