Portrait of Mariana of Austria is a 1652–1653 oil-on-canvas painting by Diego Velázquez, the leading artist of the Spanish Golden Age, existing in a number of versions. Its subject, Doña Mariana (known as Maria Anna), was the daughter of Emperor Ferdinand III and Maria Anna of Spain. She was nineteen years old when the painting was completed. Although described as vivacious and fun-loving in life, she is given an unhappy expression in Velázquez's portrait. The portrait is painted in shades of black and red, and her face is heavily made up. Her right hand rests on the back of a chair, and she holds a lace handkerchief in her left hand. Her bodice is decorated with jewellery, including a gold necklace, bracelets and a large gold brooch. A clock rests on scarlet drapery behind her, signifying her status and discernment.

Mariana had been betrothed to her first cousin, Prince Baltasar Carlos. He died in 1646 aged sixteen, and in 1649 she married her uncle, Baltasar Carlos's father, Philip IV, who sought her hand so as to preserve the hegemony of the Habsburg dynasty. She became queen consort on their marriage, and after her husband's death in September 1665, regent during the minority of her son, Charles II, until he came of age in 1675. Owing to Charles' inhibiting physical weaknesses, she dominated the political life in Spain until her death in 1696.

Velázquez completed a series of portraits of the Spanish royal family in the 1650s. The paintings are marked by an emphasis on bright hues against dark backgrounds, extravagant head-dress, and fashionably wide dresses. The series culminates with the 1656 Las Meninas, which includes Mariana and, at center-stage, her daughter the Infanta Margarita Teresa. Three full-length versions of the Portrait of Mariana of Austria survive, as well as half-length variants. The version now in the Museo del Prado is known to be the original, having been in the Spanish royal collection since its completion. Its date is based on a matching description of a canvas sent to Ferdinand in Vienna on 15 December 1651.

Mariana was born on 21 December 1634 in Wiener Neustadt, Austria, as the second child of Ferdinand III, Holy Roman Emperor, and Maria Anna of Spain, the sister of Philip IV of Spain. Maria had six children, of whom only Mariana and two sons survived into adulthood: Ferdinand (1633–1654), and Leopold (1640–1705), who became emperor in 1658. Philip's first wife, Elizabeth of France, died aged 41 in 1644. Their only son, Balthasar Charles, died of smallpox in October 1646, just a few months after his betrothal to Mariana, then 13. His death left the Spanish king heirless.

Realising that the hegemony of the Habsburg dynasty was at stake, Philip proposed marriage to Mariana, his niece. Mariana was described by art historian Rose-Marie Hagen as a "ruddy-cheeked, naive girl who loved a good laugh", and her day-to-day courtly duties came to weigh heavily on her, not least the pressure to produce a male heir.

The marriage was initially viewed as a success by the royal court when Mariana gave birth to a daughter, the Infanta Margaret Theresa. They also had two sons: Felipe Próspero was the original heir to the throne, but died in 1661 aged 3 years; Charles, the future Charles II of Spain, was born later that year.

Mariana had a difficult life. She and Philip did not know each other before their marriage, and found little in common. The marriage presented many difficulties for the royal court. He was over 40, she was 19, and her bid to provide Philip with a male heir in a family whose sons tended to be sickly, included several false hopes and miscarriages. When Philip died in 1665 she became regent for Charles, the last of the Spanish Habsburgs. Charles was infirm throughout his life, suffering from mental and physical disabilities, and the Spanish monarchy allowed women to play a powerful role in government, so Mariana was able to dominate the political life of Spain until her own death in 1696.

Velázquez sought to reinvigorate 16th-century court portraiture, which was then, according to art historian Javier Portús, "petrified into a rigid format ... with its clichés of gesture and deportment". As an official court portrait, the painting adheres to convention, with every attempt made to convey a sense of Mariana's majesty. Her extravagant taste in clothes and jewellery is evident, but a modern view is that she was a rather plain looking woman in an unhappy marriage, perhaps lacking in much of the elegance that Velázquez attributed to her.