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The city of Athens has faced air pollution problems over the last few decades due to the high population density associated with an intense emission load constrained by the local topography causing poor ventilation. In addition, the high levels of solar irradiation, in combination with emissions of nitrogen oxides and hydrocarbons, have led to the formation of photochemical pollutants, especially ozone. In this study, the evolution of the seasonal variation of primary and secondary photochemical urban air pollution in Athens was examined for the 2001–2021 period. For this purpose, the monthly NO_x (NO+NO₂), ozone, and O_x (O₃+NO₂) averages for the Athens air pollution monitoring stations of Geoponiki, Liossia, Ag. Paraskevi, and Thrakomakedones in the selected periods of 2001–2004, 2010–2013, 2016–2019, and 2020–2021 were plotted. In addition, summer diurnal NO_x, ozone, and O_x variations were examined. There was a clear reduction in NO_x levels at all four examined stations, especially at the urban background site of Geoponiki, as well as at the most polluted peripheral site of Liossia, following a reduction in urban emissions, mainly from car traffic during the examined period. The ozone and O_x curves did not show the same patterns due to the complicated nature of local ozone photochemical production, in combination with the rural surface ozone background, which was particularly high over the area.