A magnitude homogeneous earthquake catalogue spanning the twentieth century has been assembled for Greece and adjacent areas. Data assemblage had to consider all of the common magnitude scales: MW, MS, mb, and ML (including ML(ATH) of the National Observatory of Athens (NOA)). Fourteen different correlation equations between magnitude scales were considered in order to approach magnitude homogeneity in the final catalogue. This catalogue contains an estimate of both MW and MS, for each earthquake, and the actual reported values of mb, MS, MW, and ML. The final catalogue includes 5198 earthquakes after having been truncated at 4 MW to eliminate large numbers of patchily reported microearthquakes. These 5198 earthquakes during 1900–1999 are within the area 33–43°N, 18–30.99°E, focal depths 1.0–215 km, and magnitude range 4.0≤MW≤7.7. The surface wave magnitude MS is always estimated for each earthquake to facilitate application of the catalogue to seismic hazard assessment, since seismic attenuation laws are more often expressed in terms of MS rather than MW. Alongside this new catalogue, the data of Papazachos and Papazachou for the period 550 BC–1899 then provide a full picture of the known seismicity of Greece.