Garikoitz Aspiazu Rubina, also known as Txeroki, is suspected having masterminded a bomb attack on Madrid's Barajas airport in December 2006. He was arrested in the town of Cauterets, police said.
The Spanish prime minister welcomed his arrest saying: "There is no doubt that ETA has suffered a serious blow."
Jose Luis Rodriguez Zapatero added: "Although it hasn't lost the capacity to stage terror attacks...and bring pain and sorrow, its position has been weakened, and the position of the state has been strengthened."
In May 2008, French police arrested a suspected leader of ETA's political wing, Francisco Javier Lopez Pena, also known as Thierry.
Earlier on Monday, police in Belfast, the capital of Northern Ireland, arrested 53-year-old former ETA member Juan Ignacio de Juana Chaos after he gave himself up to a court.
ETA, listed as a terrorist organization by Spain, France, the United States and the European Union, has been fighting for an independent Basque state in northern Spain and southwestern France for nearly half a century.
Since 1968, ETA has been blamed for the deaths of more than 800 people and the abduction of dozens more.
