King Juan Carlos attended a ceremony at a memorial commemorating the blasts considered the deadly terrorist bombing on European soil, El Pais reported online.
Tuesday was the fourth anniversary of the bombings carried out by Muslim radical groups on the behalf of al-Qaida.
Last October, a Spanish court convicted 21 of 28 defendants in the case on various charges, including murder and possession of weapons.
The 2004 bombing came three days before Socialist Prime Minister Jose Luis Rodriguez Zapatero was elected. Zapatero, who was re-elected last weekend, had pledged to pull Spanish troops from Iraq and soon after his election made good on his vow.