The Louvre Castle (French: Château du Louvre), also referred to as the Medieval Louvre (French: Louvre médiéval), was a castle (French: château fort) begun by Philip II of France on the right bank of the Seine, to reinforce the city wall he had built around Paris. Over time, it was expanded but was generally demolished above ground in stages between 1528 and 1660 to make way for expanding the Louvre Palace, while some parts of the castle's foundations were incorporated in construction of the later palace.

The semantic distinction between a Louvre "castle" in the Middle Ages and "palace" in the modern era is no more than a recent usage. The medieval complex was often referred to simply as the Louvre (French: le Louvre), as its modern successor still is. It was also qualified in medieval times as a fortress (French: forteresse du Louvre, royal abode (French: demeure royale du Louvre), or mansion (French: hostel du Louvre or manoir du Louvre). Conversely, the Louvre Palace was still referred to as a castle (French: château du Louvre) until the mid-18th century if not later.

Before his departure for the Third Crusade in 1190, King Philip II wanted to protect his capital Paris against invasions, especially from English-held Normandy less than 100 km away, with memories still lingering of the Viking siege of Paris in 845. He ordered the construction of a new city wall, known since as the Wall of Philip II Augustus, which was started in 1190 on the right bank. The fortress was nearly squared in plan (78 m by 72 m), surrounded by a 10 m wide moat filled with water from the nearby Seine river. It was reinforced by ten defensive towers, on the corners and at the middle of each side, with twin towers defending narrow gates on the southern and eastern sides, protected by drawbridges. Two additional buildings housing the garrisons and the arsenals were located outside of the surrounding wall, to the west and south of the central courtyard, respectively.

A keep named the "big tower" (Grosse Tour du Louvre) was built around 1200 in the center of the courtyard. It was a circular structure with a diameter of 15.6 m and 30 m tall, with walls 4.25 m thick at the base. It was surrounded by a ditch, 9 m wide and 6 m deep. This ditch was dry (not a water-filled moat) and paved with large irregular stones. It was crossed by a drawbridge, whose interior arch was built in stone to limit the risk of fire. The keep had a conical roof slate over the machicolation. It also had a well and a large tank for supporting long sieges, as well as a chapel inside. The choice of a round rather than squared or rectangular keep was for military reasons, because attackers could more easily sap the wall at the angles of squared towers compared to circular towers.

The Louvre evolved gradually away from its initial purely military function. Louis IX had new rooms built in 1230–1240 without any real defensive purpose, including a ceremonial room that was later known as the Salle Saint-Louis. The Louvre became a residence intermittently during the troubled times of the 14th century. By the mid-14th century, Paris had grown well beyond the walls of Philip II. Étienne Marcel had started building a new city wall further to the west, which King Charles V the Wise (1364-1380) brought to completion, later known as the Wall of Charles V. Soon after becoming king, Charles initiated a major transformation of the Louvre into a prestigious royal residence. His architect Raymond du Temple [fr] added upper floors, windows, turrets, sculpted decoration, and gardens. Charles V repurposed the northwest tower, formerly known as Tour de la Fauconnerie (Falconry), into the first French Royal Library containing over nine hundred manuscripts.

During the Hundred Years' War, English soldiers commanded by Henry V of England, who was allied to the Burgundians who controlled Paris, entered the city. In December 1420, the English occupied the Louvre Castle without a fight. There, they found a Paris ruined by civil war and scarcity and stayed there until 1436.

In 1525, Francis I of France was defeated at Pavia and held prisoner. During his captivity, the court interfered with the King's decisions using its droit de remontrance (French for right of remonstrance). In addition, the faculty of theology and the Parlement of Paris started to show some independence. The King held a lit de justice on 24, 26 and 27 July 1526, during which he demonstrated his authority and decided to take back his kingdom and make the Louvre castle his main residence in Paris. As a symbol of his authority, he ordered the demolition of the dungeon in 1528 in order to build an Italian style palace. In 1546, he charged the architect Pierre Lescot to build a modern palace in the spirit of the Renaissance architecture, with a large hôtel particulier and ceremonial rooms. After the death of Francis in 1547, his son Henry II of France continued the work by Pierre Lescot. Between December 1546 and March 1549, he had the west wall destroyed to build a ballroom and the south wall to erect the royal pavilion (1553–1556), which housed the royal apartments and the small gallery.

After the death of Henry II, his widow Catherine de' Medici continued the development of the south wing for her apartments. From 1564 onwards, she prioritised the construction of the new Tuileries Palace and the establishment of a large Renaissance garden.