Philip I (22 July 1478 – 25 September 1506), known as Philip the Handsome or the Fair, was the first Habsburg King of Castile. The son of Maximilian I, Holy Roman Emperor, Philip inherited the greater part of the Duchy of Burgundy and the Burgundian Netherlands (as Philip IV) from his mother, Mary of Burgundy, and briefly succeeded to the Crown of Castile as the husband of Queen Joanna of Castile. He was the first Habsburg monarch in Spain. He never inherited his father's territories, nor became Holy Roman Emperor, because he predeceased his father. Having, as a young prince, met Philip the Handsome at the court of Henry VII, the future King Henry VIII of England regarded him as providing a model of leadership towards which he aspired.

Philip was born in Bruges, in the County of Flanders (today in Belgium) and was named after his great-grandfather, Philip the Good. In 1482, upon the death of his mother Mary of Burgundy, he succeeded to her Burgundian possessions under the guardianship of his father. A period of turmoil ensued which witnessed sporadic hostilities between, principally, the large towns of Flanders (especially Ghent and Bruges) and the supporters of Maximilian. During this interregnum, Philip became caught up in events and was even briefly sequestered in Bruges as part of the larger Flemish campaign to support their claims of greater autonomy, which they had wrested from Mary of Burgundy in an agreement known as the Blijde Inkomst or Joyous Entry of 1477. By the early 1490s, the turmoil of the interregnum gave way to an uneasy stand-off, with neither French support for the cities of the Franc (Flanders), nor Imperial support from Maximilian's father Frederick III proving decisive. Both sides came to terms in the Peace of Senlis in 1493, which smoothed over the internal power struggle by agreeing to make the 15-year old Philip prince in the following year.

In 1494, Maximilian relinquished his regency under the terms of the Treaty of Senlis and Philip, aged 16, took over the rule of the Burgundian lands himself, although in practice authority was derived from a council of Burgundian notables. On 20 October 1496, he married Infanta Joanna, daughter of King Ferdinand II of Aragon and Queen Isabella I of Castile, in Lier, Belgium. The marriage was one of a set of family alliances between the Habsburgs and the Trastámara, designed to strengthen against growing French power, which had increased significantly thanks to the policies of Louis XI and the successful assertion of regal power after war with the League of the Public Weal. The matter became more urgent after Charles VIII's invasion of Italy (known as the First Peninsular War).

Philip's sister Margaret married John, Prince of Asturias, only son of Ferdinand and Isabella and successor to the unified crowns of Castile and Aragon. The double alliance was never intended to let the Spanish kingdoms fall under Habsburg control. At the time of her marriage to Philip, Joanna was third in line to the throne, with John and his sister Isabella married and hopeful of progeny.

In 1500, shortly after the birth in Flanders of Joanna and Philip's second child (the future Emperor Charles V), the succession to the Castilian and Aragonese crowns was thrown into turmoil. The heir apparent, John, had died in 1497 very shortly after his marriage to Margaret of Austria. The succession thereby passed to Queen Isabella and King Manuel I of Portugal. She died in 1498, while giving birth to a son, the Infante Miguel, to whom succession to the united crowns of Castile, Aragon and Portugal now fell; however, the infant was sickly and died during the summer of 1500. The succession to the Castilian and Aragonese crowns now fell to Joanna and Philip. Because Ferdinand could produce another heir, the Cortes of Aragon refused to recognise Joanna and Philip as the heirs presumptive to the Kingdom of Aragon. In the Kingdom of Castile, however, the succession was clear. Moreover, there was no Salic tradition which the Castilian Cortes could use to thwart the succession passing to Joanna. At this point, the issue of Joanna's mental incompetence moved from courtly annoyance to the centre of the political stage, since it was clear that Philip and his Burgundian entourage would be the real power-holders in Castile.

In 1502, Philip, Joanna and a large part of the Burgundian court travelled to Spain to receive fealty from the Cortes of Castile as king, a journey chronicled in intense detail by Antoon I van Lalaing (French: Antoine de Lalaing), the future Stadtholder of Holland and Zeeland. Philip and the majority of the court returned to the Low Countries in the following year, leaving a pregnant Joanna behind in Madrid, where she gave birth to Ferdinand, later Holy Roman Emperor. Philip's life with Joanna was rendered extremely unhappy by his infidelity and political insecurity, during which he consistently attempted to usurp her legal birthrights of power. This led in great part to the rumors of her insanity due to reports of depressive or neurotic acts committed while she was being imprisoned or coerced by her husband; most historians now agree she was merely clinically depressed or schizophrenic at the time, not insane as commonly believed. Before her mother's death, in 1504, husband and wife were already living apart.

In 1504, Philip's mother-in-law died, leaving the Crown of Castile to Joanna and Philip I. Isabella I's widower and former co-monarch, King Ferdinand V, endeavoured to lay hands on the regency of Castile, but the nobles, who disliked and feared him, forced him to withdraw. Philip was summoned to Spain, where he was recognized as king. He landed, with his wife, at La Coruña on 28 April 1506, accompanied by a body of German mercenaries. Father and son-in-law mediated under Cardinal Cisneros at Remesal, near Puebla de Sanabria, and at Renedo, the only result of which was an indecent family quarrel, in which Ferdinand professed to defend the interests of his daughter, who he said was imprisoned by her husband. A civil war would probably have broken out between them; but Philip, who had only been in Spain long enough to prove his incapacity, died suddenly at Burgos, apparently of typhoid fever, on 25 September 1506. His wife supposedly refused to allow his body to be buried or to part from it for awhile. Philip I is entombed at the Royal Chapel of Granada (Capilla Real de Granada), alongside his wife, and her parents Isabella I and Ferdinand II.