Police cordoned off both towns and a bomb exploded near Laredo's seafront.
Holidaymakers had been cleared from the beach 45 minutes earlier and took cover in local cafes and bars which drew down shutters to protect against the blast, witnesses said.
"There were no injuries because the area had been cleared and cordoned off," a national government spokesman said.
Later another bomb exploded among sand dunes at Noja.
A fourth bomb exploded at a golf course near Noja after the area had been cleared by police, Spain's La Ser radio station reported.
Spain's ruling Socialist party quickly condemned the attacks in a statement.
"We want to show our solidarity with the area's inhabitants and its visitors," it said. "Once again, Eta is attacking all citizens."
Eta declared a unilateral ceasefire in March 2006, raising hopes for an end to the violence.
But an Eta bombing at Madrid's airport in December 2006 that killed two men put an end to tentative peace talks with the government.
The group officially terminated the ceasefire in June last year, and since then the Spanish government has blamed it for four killings including civil guards across the French border.