Eleanor of Toledo (Italian: Eleonora di Toledo (1522 – 17 December 1562), born Doña Leonor Álvarez de Toledo y Osorio, was a Spanish noblewoman who was Duchess of Florence from 1539, after Margaret of Austria.Although, Eleanor is often referred to as the Grand Duchess of Tuscany, she predeceased the creation of the Grand Duke of Tuscany. She is credited with being the first modern first lady, or consort. She served as regent of Florence during the absence of her spouse.

Eleanor was born in Alba de Tormes, Salamanca, as the second daughter of the Viceroy of Naples, Don Pedro Álvarez de Toledo, Marquis of Villafranca (Charles V's lieutenant-governor) and Maria Osorio, 2nd Marquise of Villafranca. Her father was the second son of Fadrique Álvarez de Toledo, 2nd Duke of Alba and therefore, the 3rd Duke of Alba was his eldest brother.

Eleanor was remembered for her beauty: brunette with hazel eyes, an oval shaped face, sweet and full features of an inherent majestic quality, as evident in her portraits. Eleonor was seen as an incredibly charming bride entering the Medici household.

Cosimo was searching for a wife who could help strengthen his political position and had initially asked to marry Margaret of Austria, the widow of Duke Alessandro de’ Medici, who displayed enormous reluctance at the idea (thus serving her father’s own interest and plans for her). The Emperor, not wanting to antagonize Cosimo, offered him one of the daughters of the rich viceroy of Naples. The viceroy agreed, provided Cosimo settled a large amount of money on her as dowry. This marriage was politically beneficial as the Medici were new to their ducal status. Additionally, Eleanor's royal Castilian ancestors and relations with the Habsburgs provided the Medici with the blue blood they had hitherto lacked and began the process of placing them on a footing with other European sovereigns. Through her father, Eleanor also provided the Medici with a powerful link to Spain, at that time ultimately in control of Florence, so that the marriage offered Cosimo I the opportunity to show sufficient loyalty to and trust in Spain that Spanish troops could be withdrawn from the province.

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