In 1521 aged nineteen, he succeeded his father and became King of Portugal. On 5 February 1525 at Salamanca, he married Archduchess Catharina of Austria, a sister of his step-mother, and they became the parents of nine children. Joao III is generally represented as possessing a limited intellect. A religious fanatic of the worst kind, he introduced the Inquisition and the Jesuits into Portugal, and occupied himself with the founding of new Bishoprics. Although it was not apparent on the surface, his reign was an era of decline.
Of his nine children, the only one to survive was a sickly son, Joao, who in 1552 was married at the age of fifteen to his double first cousin Juana, daughter of the Emperor Charles V. However,
leaving a pregnant wife, Joao died aged sixteen. King Joao III never fully recovered from the loss of his last child and, on 11 June 1557, died of apoplexy, a few days after his fifty-fifth birthday.
 

Source: Leo van de Pas