No one was hurt as police had received a telephoned warning in time to clear the area.
Although most of the damage was inside the Socialist party office, the blast was strong enough to buckle nearby cars and shatter windows.
"A bomb placed by ETA exploded at 0325 (0125 GMT) this morning ... at the Socialist party office in Elgoibar [Guipuzcoa province]," a police statement said.
The attack was similar to a bombing blamed on ETA on Thursday outside a Socialist office in the city of Bilbao, which wounded seven police officers.
ETA has killed more than 800 people in four decades of armed struggle for independence for the Basque Country from Spain.
The guerrilla group's last fatal attack was on March 7 when, two days before a general election, it shot dead a former Socialist Party councillor in the Basque town of Mondragon. (Reuters)
Bomb explodes in Spain's Basque region