This portrait is one of a pair depicting Isabella I of Castile (1451-1504) and her husband Ferdinand II of Aragon (1452-1516), King and Queen of Spain, and parents of Katherine of Aragon (1485-1536). The paintings were first recorded in the Royal Collection during the reign of Henry VIII; however, it is possible that the portraits came to England at the time of Katherine's marriage to Prince Arthur, Henry's elder brother or were sent as diplomatic gifts during the negotiations for the match.

Isabella I was hereditary Queen of Castile in her own right and her marriage in 1469 to her cousin, Ferdinand II of Aragon united their kingdoms and transformed Spain into a major European power, paving the way for unification under their grandson, Holy Roman Emperor Charles V. During their joint reign, Ferdinand and Isabella expanded their territories, financing Christopher Columbus in his 1492 voyage, which resulted in discovery of the Americas. For their fervent defense of the Catholic faith, they were proclaimed 'The Catholic Kings' by Pope Alexander VI.

This portrait shows Isabella I in three-quarters against a blue background, wearing a lavishly embroidered cloth of gold dress, white skull-cap and double-strand gold necklace with a ruby and pearl pendent. The jewel is possibly the 'un rubi balax grande e quartro perlos en el puestas grandes' described in an inventory of 1453. She holds a closed book in both hands, with the thumb of her right hand marking a page. The holding of a book, commonly a religious text, was used to represent the piety of a sitter during this period and was particularly associated with portraits of women.

This portrait is strikingly similar to another version of this composition, dated c.1490, in the Museo del Prado, Madrid. The Prado portrait was formerly in the Capesthorne Collection and is the reverse of that in the Royal Collection. It appears to shows Isabella at a slightly younger age and it is possible that it may have been sent to potential husbands during marriage negotiations. It has been suggested that the painting may be by an English artist who travelled to the Spanish court with the English Ambassador. However, details of the painting indicate a possible Flemish origin and it is currently thought to be by an anonymous Flemish artist.

Unlike other known versions of the portrait, the Royal Collection painting is paired with a portrait of Ferdinand II of Aragon (RCIN 403448). In it, the King is shown in three-quarters against a blue background, wearing a richly embroidered robe made from the same cloth of gold seen in the portrait of his wife. Additionally, in both portraits a stone sill is painted in front of the sitters on which their names are inscribed. Additionally, mid-twentieth century conservation examination revealed both paintings to be round-headed and of exactly the same size. It is probable therefore that the portraits were painted as a pair by the same artist, although it is not currently possible to say who that artist was.

The painting appears in Pyne's illustrated 'Royal Residenes' of 1819, hanging in The Old Drawing Room at Kensington Palace (RCIN 922153).