Juan, Conde de Montizon (May 15, 1822 - November 21, 1887) was the younger son of the Infante Carlos of Spain. He was the Carlist claimant to the throne of Spain, and later the Legitimist claimant to the throne of France.

Juan Carlos Maria Isidro, Infante of Spain, was born at the Palacio Real de Aranjuez in 1822. His parents were the Infante Carlos, brother of King Ferdinand VII, and his first wife, the Infanta Francisca of Portugal. He was raised in an atmosphere imbued with traditional values of loyalty to the monarchy and the Church.

In 1833 Juan's father claimed the throne of Spain as King Carlos V. On account of his youth Juan played little part in the campaigns of the ensuing First Carlist War.

On February 6, 1847, Juan married the Archduchess Beatrix of Austria-Este, daughter of Duke Francis IV of Modena. The couple had two sons:

• Carlos, Duke of Madrid (1848-1909).
• Alfonso Carlos, Duke of San Jaime (1849-1936).

Juan and Beatrix lived first in Modena, but had to leave during the revolution of 1848. After a brief time in Austria, they settled in London where their younger son was born. In spite of the conservativism and religious piety of his own family and that of his wife, Juan developed liberal tendencies. He separated from his wife who returned to Modena where she raised her two sons.

Juan played no part in the 1860 Carlist rising led by his brother Carlos, Conde de Montemolin. On April 21 Carlos was captured by the troops of Isabella II and forced to renounce his claims to the Spanish throne. On June 2 Juan published a declaration affirming his accession as Juan III, King of Spain; henceforward he used the title conde de Montizón (in commemoration of a commandery of the Order of Santiago which belonged to his father). Juan's accession declaration used phrases such as "the light and progress of the age"; these phrases caused great offence to many Carlists most of whom refused to support him.

Once he had left Spain, Carlos renounced his abdication. On June 15 he declared that it was invalid since he had been forced to sign against his will. Juan refused to accept his brother's declaration. Until Carlos' death the following January there were two Carlist claimants.

During the early 1860s the popularity of the government of Isabella II continued to decline. Juan's liberal views, however, ensured that he was not a viable candidate for the Carlists. In 1866, Juan's elder son Carlos (now aged eighteen) asked his father to abdicate his rights, but he did nothing. Two years later, however, on October 3, 1868 Juan signed a decree of abdication at Paris. He became an active supporter of his son Carlos' attempts to regain the Spanish throne in the Third Carlist War .

After his abdication Juan lived mostly in England in the town of Hove near Brighton. He used the name Mr. Montagu. He lived with an Englishwoman with whom he had two daughters.

On August 24, 1883 Juan's distant cousin Henri, comte de Chambord died. Henri had been the Legitimist claimant to the throne of France. Henri's widow and a minority of his supporters held that Juan as senior male descendant of Louis XIV was his successor. They proclaimed him as Jean III, King of France and Navarre. He issued a declaration saying, "Having become Head of the House of Bourbon by the death of my brother-in-law and cousin the comte de Chambord, I declare that I do not in any way renounce the rights to the throne of France which I have held since my birth". But other than this declaration he made no active claim to the French throne.

Juan died at his home (25 Seafield Road) in Hove in 1887. His funeral mass was held November 24 in Sacred Heart Church in Hove in the presence of his two sons. Then his body was taken to Trieste where it is buried in the chapel of Saint Charles Borromeo in the Basilica di San Giusto.