Mariano Téllez-Girón y Beaufort Spontin, 12th Duke of Osuna, GE, OM, LH, OAN, KA (19 July 1814 – 2 June 1882), was a Spanish peer, diplomat and army officer, whose lavish exploits as Ambassador of Spain to the Russian Empire earned him admiration and popularity amongst European courts.

The Duke of Osuna, at the time styled Marquess of Terranova, saw action in many fronts of the First Carlist War, being profusely decorated in July 1836.

The duke, "a formidable dandy",[3][4][5][6] was ultimately sent to recover the prestige of a decadent Spain in the complex Tsarist court, influenced at the time by the English and French envoys.

[7] During his tenure as ambassador in Saint Petersburg, he achieved the resumption of strong diplomatic relations between the Kingdom of Spain and the Russian Empire, which had been broken at the death of Ferdinand VII.

As claimed by Valera and Béthencourt, in one occasion, the Duke of Osuna celebrated an opulent dinner in which he ordered that the guests threw the entirety of the tableware of gold through the windows of his palace and into the River Neva, so as to spare washing effort to his servants.

On 29 August 1844 he inherited the titles and estates of the House of Osuna from his brother, and began a life of luxury that ended with his great ruin.

He was the last effective lord of all the family estates that were confiscated by the laws of Mendizábal, which they used that upon his death without succession, and since his main heir was the Duke of Alba (who had already accumulated 7 Grandeeships), a huge lawsuit was initiated.