Castel del Monte (Italian for "Castle of the Mountain") is a 13th-century citadel and castle situated on a hill in Andria in the Apulia region of southeast Italy. It was built during the 1240s by the Emperor Frederick II, who had inherited the lands from his mother Constance of Sicily. In the 18th century, the castle's interior marbles and remaining furnishings were removed. It has neither a moat nor a drawbridge and some considered it never to have been intended as a defensive fortress; however, archaeological work has suggested that it originally had a curtain wall. Described by the Encyclopaedia Italiana as "the most fascinating castle built by Frederick II", the site is protected as a World Heritage Site. It also appears on the Italian version of the one-cent euro coin.

Castel del Monte is situated on a small hill close to the monastery of Santa Maria del Monte, at an altitude of 540 m. When the castle was built, the region was famously fertile with a plentiful supply of water and lush vegetation. It lies in the comune of Andria, occupying the site of an earlier fortress of which no structural remains exist.

The castle's construction is mentioned in only one contemporary source, a document dating to 1240, in which the Holy Roman Emperor Frederick II ordered the governor of Capitanata to finish some works in it. It was never finished and there is no proof that the emperor used it as a hunting lodge as commonly stated. It was later turned into a prison, used as a refuge during a plague, and finally fell into disrepair. It originally had marble walls and columns, but all were stripped by vandals or re-used in constructions nearby.

Because of its relatively small size, it was once considered to be no more than a "hunting lodge", but scholars now believe it originally had a curtain wall and did serve as a citadel. Frederick was responsible for the construction of many castles in Apulia, but Castel del Monte's geometric design was unique. The fortress is an octagonal prism with an octagonal tower at each corner. The towers were originally some 5 m higher than now, and they should perhaps include a third floor. Both floors have eight rooms and an eight-sided courtyard occupies the castle's centre. Each of the main rooms has vaulted ceilings. Three of the corner towers contain staircases. The castle has two entrances, an unobtrusive service entrance and an ornate main entrance. Frederick's main entrance featured elements from classical design, and may have been influenced by Frederick's interest in Greco-Roman architecture.

The octagonal plan is unusual in castle design. Historians have debated the purpose of the building and it has been suggested that it was intended as a hunting lodge. Another theory is that the octagon is an intermediate symbol between a square (representing the earth) and a circle (representing the sky). Frederick II may have been inspired to build to this shape by either the Dome of the Rock in Jerusalem, which he had seen during the Sixth Crusade, or by the Palace Chapel of Aachen Cathedral.

Occasionally used as a hunting lodge under Manfred of Sicily, the castle become a state prison under the latter's victor, Charles I of Anjou: here Manfred's sons Henry, Azzo and Enzo were kept as prisoner after 1266, as well as other Hohenstaufen supporters.

The main wall is 25 m high and the eight bastions each 26 m. The sides of the main octagon are 16.5 m long and those of the octagonal towers each 3.1 m. The castle has a diameter of 56 m. Its main entrance faces east.

The Italian design of the 1-cent euro coin is a depiction of the Castel del Monte.

In the 18th century, the castle's marbles and other ornamentation were looted. Members of the House of Bourbon took the marble columns and window frames and reused them at their palace in Caserta. What remains now includes fragments of a knight and a re-used Roman relief, while in the Provincial Gallery of Bari there are a head fragment and a cloaked, headless bust, sometimes interpreted as Frederick II.  After having been abandoned for a considerable length of time, the castle was purchased in 1876 for the sum of 25,000 lire by the Italian State, which began the process of restoration in 1928.

In 1996 Castel del Monte was named a World Heritage Site by UNESCO, which described it as "a unique masterpiece of medieval military architecture".