The ages of those who died are not yet known.
Two other people were taken to hospital, including one person whose condition was described as critical, the mayor of Villafranca de Ebro told Spanish radio.
Volga Ramírez told Cadena Ser that the tragedy may have been caused by a mattress on fire in one of the rooms.
Witnesses said there was little fire damage to the single-storey building and smoke inhalation is being blamed for the large number of fatalities.
“My husband went in to get people out. You couldn’t even breathe because of the smoke,” the mayor told reporters.
The Spanish government’s delegate in Aragón region, Fernando Beltrán, paid his condolences to the victims and said they would continue to monitor the progress of those in hospital and the investigation into the cause of the fire.
Prime Minister Pedro Sánchez also expressed his dismay and shock at the disaster.
Nine years ago a fatal fire left nine people dead at another retirement home in the Zaragoza area.
