Tuna trawler and crew released amid reports of $3.5m ransom having been paid by Madrid.
Navfor, the European Union naval force operating in the area, confirmed on Tuesday that pirates had seized the 22,294 DWT tanker.
The ship, which operates out of Singapore, had been sailing to the Kenyan port of Mombasa but had turned round after being seized near the Indian Ocean archipelago.
Navfor also said on Wednesday that the MV Maersk Alabama, a US-flagged container ship which was hijacked by Somali pirates seven months ago, had been attacked for a second time.
The naval force said the crew returned fire after pirates fired at the ship, about 650km east of Somalia. No casualties were reported.
Attacks off the Seychelles have surged after pirates extended their range to evade navies patrolling off the Horn of Africa.
On Tuesday, Somali pirates freed a Spanish tuna fishing boat hijacked six weeks ago in the Indian Ocean, with a pirate saying that a $3.5m ransom had been paid for the Alakrana and its crew of 36.