Juan Martínez Silíceo (1486–1557) was a Spanish Roman Catholic bishop, cardinal and mathematician.
Juan Martínez Silíceo was born in Villagarcía de la Torre in 1486, the son of Juan Martínez Guijeno, a poor laborer, and Juana Muñoz. His last name is also given as Guijarro.
Martínez studied grammar in Llerena, a small town near Villagarcía de la Torre, and then studied philosophy in Seville. To support himself, he served as a sacristan in the parish church in his home town. He then became the tutor of two sons of a gentleman in Valencia. He then moved on to the University of Paris to complete his studies. Afterwards, he became professor of moral philosophy at the Colegio de San Bartolomé, University of Salamanca.
After he was ordained as a priest, he became a professor of Christian theology. He then became the canon theologian of the cathedral chapter of the Cathedral of Coria. In July 1534, he was named tutor to Philip, Prince of Asturias, later becoming the prince's almoner and confessor.
On February 23, 1541, he was elected Bishop of Cartagena; he was consecrated as a bishop later that year. (He did not, however, enter his see until 1544.) In 1543, Prince Philip despatched him to Badajoz to retrieve the prince's fiancée, Maria Manuela, Princess of Portugal. On January 8, 1546, he was promoted to the metropolitan see of Toledo.
Pope Paul IV made him a cardinal priest in the consistory of December 20, 1555. He received the red hat and the titular church of Santi Nereo e Achilleo on February 1, 1556. He became a renown mathematician for his time after publishing "Arithmética", firstly printed in Paris and afterwards all across Spain. Apart from "Arithmética" he also publish "Arte calculatorio".
He died in Toledo on May 31, 1557. He was buried in the church of the Colegio de Doncellas Nobles in Toledo.