Spanish authorities say Mikel Irastorza is the "currently the maximum leader" of ETA. They say he was arrested this morning in the French town of Ascain near the Spanish border.
In 2011 ETA announced it was renouncing the use of violence in its campaign for independence for the Basque regions of northern Spain and southwest France.
The militant group, which is blamed for over 800 deaths since its first killing in 1968, has not disbanded.
French police are continuing their investigations into the group and don't rule out more arrests.