A descendant of Britain's James II, the noblewoman is said to be able to cross the country from north to south without leaving her estates and has more titles than the Queen.
Her fiancé comes from much more modest stock. Alfonso Diez Carabantes is a civil servant and long-standing family friend. At 61, he is 24 years her junior.
Opposition to the match came from some of her six children. But true love won through in the end, and the nuptials are reportedly scheduled to go ahead in September.
The duchess – full name Maria del Rosario Cayetana Alfonsa Victoria Eugenia Francisca Fitz-James Stuart y de Silva – has outlived two husbands.
She grew close to Alfonso after her second – free-thinking former Jesuit priest Jesus Aguirre y Ortiz de Zarate – died in 2001. (Her first marriage to the scion of a noble family lasted 25 years, ending with his death in 1972).
In 2008, the noblewoman, known as Cayetana, spoke to HELLO! magazine's sister publication HOLA! about why she was thinking about marrying again.
"I felt very lonely after Jesus died, and I developed feelings for Alfonso," she told HOLA!
"When you get to know someone and you like them, you end up falling in love a little, and I fell in love with him.
"At one point, he confessed that he had developed feelings for me over the years. At first, I thought he was crazy.
"Later it hit me – it was something that would fill any woman with happy pride."
The couple made plans for their future, but not everyone was as thrilled about the romance.
"The tough part was that my children didn't understand and they got quite angry with me," she explained.
"It's true that I planned to marry. We were both full of enthusiasm for the idea. I took a step back for my children.
"I saw that everything was going to be very complicated."
But the determined aristocrat – who is a constant presence at Spanish society events despite her increasing fragility – was not to be deterred.
Earlier this summer, she finally got her children on side by dividing up her vast wealth – thought to between 600m and 3.5bn euros (£524m and £3bn) – between them. The inheritance included a collection of estates, palaces and artworks.
Among them is a portrait by Francisco de Goya of her ancestor, the Duchess of Alba, who is said to have been the artist's lover and the model for his masterworks 'The Clothed Maja' and 'The Naked Maja'.
With sprawling residences in Ibiza, Seville and Madrid, the couple have plenty of venues at their disposal for the nuptials.
They have reportedly chosen the ancestral palace Las Dueñas in Seville (below) – the majestic 16th-century estate where Cayetana married her previous husbands and raised her six children.
A selection of the Duchess' titles
- 18th Duchess of Alba de Tormes
- 11th Duchess of Berwick
- Duchess of Arjona
- Duchess of Hijar
- 11th Duchess of Liria and Jerica
- 14th Countess-Duchess of Olivares
- Marchioness of El Carpio
- Countess of Aranda
- Countess of Lemos
- Countess of Lerin and Condestablesa of Navarra
- Countess of Miranda del Castañar
- Countess of Monterrey
- Countess of Osorno
- Countess of Palma del Rio
- 12th Marchioness of la Algaba
- Marchioness of Almenara
- 14th Marchioness of Barcarrota 19th Marchioness of Coria
- Marchioness of Eliche
- Marchioness of Mirallo
- Marchioness of Moya
- Marchioness of Orani
- Marchioness of Osera
- 11th Countess of Tinmouth
- Viscountess of la Calzada
- 11th Baroness of Bosworth
- Viscountess of la Calzada
- 29th Dame of Moguer