The bombing on the north coast was the latest in a series of attacks by ETA and may be part of a summer campaign aimed at undermining Spain's vital tourist industry.
A car parked outside the airport terminal blew up about 5 pm(1500 GMT), setting several other vehicles ablaze and sending up a plume of black smoke.
The explosion damaged the airport facade, blowing out windows and twisting metal, wrecked 12 cars and caused lesser damage to about 40 other vehicles, officials and local news reports said. Burned-out cars littered the car park.
“Once again, ETA...has tried to sow terror," grim-faced Interior Minister Angel Acebes told reporters after flying in to Santander airport to survey the damage.
"ETA is doing everything it can, with everything it's got."
Noone was injured because a caller representing ETA telephoned a warning to a Basque newspaper an hour before the explosion, giving police time to clear the area. About 60 people in the terminal were escorted to safety on the airport runway.
“Once again, ETA...has tried to sow terror."
-Spanish Interior Minister Angel Acebes
The Spanish airport authority AENA said evening departures from the airport had been cancelled while three flights heading to Santander had been diverted to Bilbao. It hoped to get flights back to normal on Monday.
ETA, branded a terrorist organisation by the United States and the European Union, has killed more than 800 people in a three-decade campaign for an independent Basque state in northern Spain and southwestern France.
Spain's tough stance
Acebes pledged Spanish security forces would continue battling ETA with firmness and determination.
He attacked the Basque regional government, whose leader, moderate nationalist Juan Jose Ibarretxe, has proposed a referendum on greater Basque self rule. Acebes accused the Basque government of giving priority to unconstitutional plans to break up Spain rather than focusing on detaining ETA members who may be operating from the Basque country.
The airport lies across a bay from Santander, a picturesque port and seaside resort that is packed with holidaymakers in summer. The city is popular with Britons as it has a direct ferry service to Plymouth in England.
The explosion came two days after Spain's High Court sentenced two ETA members, Santiago Arrospide and Rafael Caride Simon, to 790 years each in prison for their part in ETA's bloodiest attack -- the June 1987 bombing of a Barcelona supermarket which killed 21 people. Under Spanish law, they will serve a maximum of 30 years.
The last week has seen an upsurge in ETA attacks.
Two bomb blasts ripped through seaside hotels in Alicante and Benidorm last Tuesday, injuring 13 people, seven of them foreigners, in what the government said appeared to mark the start of ETA's now traditional summer campaign against tourism.
On Friday, a bomb exploded outside a Spanish courthouse, injuring a man, after a warning from ETA. Two home-made bombs blew up in the Basque country the same day.