The 15-nation council, on which Spain holds a non-permanent seat, passed a resolution naming the group as the perpetrator of the attack after diplomats said they accepted the accusation put forward by Madrid.
But previous UN resolutions in similar circumstances had stopped short of naming a specific individual or group, and several UN officials privately expressed surprise at the decision to finger ETA for the bombings.
Spain was a firm US ally in the war on Iraq and suspicion mounted later that al-Qaida may have carried out the bombings in reprisal.
French UN ambassador Jean-Marc de la Sabliere and his US counterpart John Negroponte said the council had accepted Spain's decision to blame ETA, although diplomats said other nations expressed some reservations.
Negroponte shrugged off suggestions that al-Qaida or Islamist hardliners angry over the Iraq war might have had a hand in the attacks.
"We are satisfied by the fact that the Security Council acted with such promptness to condemn unanimously this terrible terrorist attack," Negroponte said.
But officials expressed surprise at the haste with which ETA was named and pointed out that the council had not named al-Qaida in the resolution passed a day after the September 11 2001 attacks on the US.
"You get the feeling they just jumped the gun on this," said one UN official on grounds of anonymity.