The fire broke out at one of the Vandellos II power station's electricity generating units and triggered an emergency shutdown, the Nuclear Safety Council said in a statement.
The fire was extinguished two hours later, and there were no injuries or environmental damage.
Vandellos II is situated on the Mediterranean coast 140 kilometers (87 miles) southeast of Barcelona, and is one of seven Spanish nuclear plants.
It is run by Spain-based energy companies Endesa and Iberdrola, S.A.
The fire was the second incident at a Spanish nuclear plant in recent months.
The Asco plant, located along the Ebro River 44 miles (28 kilometers) upstream from the Mediterranean, leaked in November 2007, but operators did not detect it until March. They then waited until April 4 to notify regulators, according to the Nuclear Safety Council.
The council recently proposed a fine of up to 22.5 million (US$33 million) over the leak, saying operators initially downplayed the amount of contamination released.
Spain has seven nuclear power plants. The Socialist government, which has come out against nuclear energy, says it will let them run until their licenses expire, then decommission them.