Ms Chacón was found dead at her Madrid home, aged 46, after a neighbour had raised the alarm. A congenital heart problem was the likely cause of death.
She was Spain's first woman defence minister, symbolising the change of culture since the end of military rule.
Her son Miquel is eight years old.
In 2007 Ms Chacón also served as housing minister in the Socialist government led by ex-Prime Minister José Luis Rodríguez Zapatero.
Spanish politicians were among the many Spaniards who expressed shock and sympathy over Ms Chacón's sudden death.
