During the ceremony in Valencia, King Felipe VI addressed the relatives of the deceased.
"The Queen and I want you to know that we are with you all now and forever," he said.
The floods are one of Europe's worst natural disasters. 237 people died, 229 of whom were in the Valencia region.
Prime Minister Pedro Sánchez and the Valencia regional president, Carlos Mazón, also attended the ceremony.
Mazón was met with profanity and insults from family members of the victims and has been criticized for his slow and inadequate handling of the emergency.
