Don Agustin Fernando Muñoz y Sánchez (4 May 1808 – 11 September 1873), Duke of Riánsares and Montmorot, Marquis of San Agustín, was the second and morganatic husband of Maria Christina, Regent of Spain.
Muñoz was born at Tarancón, in the Province of Cuenca, in New Castile. His father was the keeper of an estanco or office for the sale of the tobacco of the government monopoly.
Muñoz enlisted in the royal bodyguard, and attracted the attention of Maria Christina. According to one account, he distinguished himself by stopping the runaway horses of her carriage; according to another, he only picked up her handkerchief; a third and scandalous explanation of his fortune has been given. Maria Christina's husband, King Ferdinand VII of Spain died on 28 September 1833, and on 29 December 1833 she and Muñoz were privately married.
If Maria Christina had officially made the marriage public, she would have forfeited the regency; but her relations with Muñoz were perfectly well known within the Spanish court. When on 13 August 1836 the soldiers on duty at the summer palace La Granja mutinied and forced the regent to grant a constitution, it was generally, though wrongly, believed that they overcame her reluctance by seizing Muñoz, whom they called her guapo, or fancy man, and threatening to shoot him. In 1840 Maria Christina found her position intolerable; she renounced the regency and left Spain with Muñoz. In 1842 Maria Christina purchased the Château de Malmaison as their residence. In 1843, on the overthrow of General Baldomero Espartero they returned to Spain.
In 1844, Muñoz's stepdaughter Queen Isabella II was declared to be of age. On 23 June 1844 Isabella gave to Muñoz the title duque de Riánsares, to which was attached a Grandeza de España; the title came from the river Ánsares, near Muñoz's birthplace in Tarancón. On October 12, 1844 Isabella gave official consent to the marriage between her mother and Muñoz, and it was publicly performed. In 1846 Isabella made Muñoz a Knight of the Golden Fleece. On 30 May 1846 she gave Muñoz a second title, marqués de San Agustín. Muñoz was made a Captain General, the highest rank in the Spanish Army. In 1847 Louis Philippe, King of the French, gave Muñoz the title duc de Montmorot; he also invested Muñoz with the Grand Cross of the Légion d'honneur.