Maria de las Mercedes de Borbon y Orleans, 89, the Countess of Barcelona who was the mother of Spain's King Juan Carlos, died Jan. 2 at her home on the Canary Island of Lanzarote. The cause of death was not reported.
The royal family had traveled to Lanzarote to celebrate the New Year at the residence.
Spanish Prime Minister Jose Maria Aznar, who was at the residence Sunday to meet the king, offered his condolences.
The countess was the widow of Don Juan de Borbon y Battemberg, onetime heir to the throne. She married in 1935 and had four children, including Juan Carlos, born in Rome in 1938. He took the throne in 1975 after an agreement between Don Juan and then-ruling Gen. Francisco Franco.
King Juan Carlos is revered for helping to orchestrate Spain's peaceful shift to democracy after Franco's death ended nearly four decades of authoritarian rule.
Born in Madrid, Maria de las Mercedes went into exile, as did the royal family, in 1934 after King Alfonso XIII gave up power and Spain was declared a republic.
When Franco launched his military uprising in 1936, it soon became clear Franco had no intention of restoring the monarchy, and Don Juan became one of his most vocal critics.
It was, however, an agreement between Franco and Don Juan that allowed Juan Carlos to be educated in Spain and eventually to become king.
Don Juan's wife stood by him in exile, living in France, Italy, Switzerland and Portugal and making only occasional visits to Spain before returning home for good in the 1970s.
The countess was known as a lover of much that was Spanish, including the country's art, music and bullfighting.
Upon learning of the countess's death, opposition Socialist party spokesman Alfredo Perez Rubalcaba highlighted "the role she played alongside Don Juan de Borbon in keeping the image of the monarchy alive during [Franco's] dictatorship, something which was fundamental during the transition" to democracy.