The atmospheric energy budget in the centre of Athens, Greece was determined during the Thermopolis 2009 campaign in order to investigate the development of the urban heat island. Heatwaves during summer are a common occurrence in this large conurbation. Micrometeorological data from a tower were acquired in a densely built central district, and net all-wave radiation, sensible heat, latent heat and momentum flux densities were derived by the eddy-covariance method and also estimated using Monin–Obukhov similarity relationships. Under the prevailing hot and dry conditions, sensible heat-flux density was on average five times larger than the latent heat-flux density. The anthropogenic contribution to the energy budget was also determined on the basis of the acquired data.