Kinship

How were Philip IV of Spain and Mariana of Austria related?

Uncle and niece: Mariana’s mother, Maria Anna of Spain, was Philip IV’s sister. They were also first cousins once removed through Charles II, Archduke of Austria and Maria Anna of Bavaria — Philip’s grandparents and Mariana’s great-grandparents.

The nearer route is short. Philip IV and Maria Anna were children of Philip III and Margaret of Austria. Maria Anna married her cousin Ferdinand III, the future emperor, in 1631, and Mariana, born in 1634, was their daughter. Counted in generations, Philip stands one step below the shared couple and Mariana two, which makes him her uncle and her his niece. The bride was an archduchess of the Austrian branch, yet this tie to the Spanish king came wholly through her mother, the infanta her father had married.

The second route runs through Philip’s mother. Queen Margaret was a daughter of Charles II, Archduke of Austria and Maria Anna of Bavaria, and a sister of the future Emperor Ferdinand II — Ferdinand III’s father. Mariana’s father and Philip were therefore first cousins, which places Mariana three generations below the archduke and his wife and Philip two: first cousins once removed. The diagram draws two lines from Mariana up to that couple. The one through her father is the new tie; the one through her mother passes through Queen Margaret herself and only retraces the uncle route.

Mariana had first been promised to Philip’s son. Her betrothal in 1646 to Balthasar Charles, heir to the Spanish crowns, ended with the prince’s death that same year, and Philip, already a widower, was left without a son to succeed him. The king took his son’s intended bride as his own second wife, marrying her in October 1649, when he was 44 and she 14. His Wikipedia biography treats the choice as political, a way to tighten Spain’s bond with Habsburg Austria. By the count in their son’s Wikipedia biography, only two marriages of Spanish monarchs between 1450 and 1661 joined an uncle to his niece, and this was one.

Of the couple’s five children, two lived to adulthood: Margaret Theresa, who went on to marry her mother’s brother Leopold I, and Charles II, the last Habsburg king of Spain. Charles was in poor health for long stretches of his life, and what caused it is still argued over. His Wikipedia biography points out that his sister, born of the same marriage, did not suffer in the same way, and notes that some modern researchers point to an infection he caught as an infant.

Computed from every recorded line in The House of Habsburg on Kinlight; written 29 September 2026.