Ujazdów Castle ([ujazduv], Polish: Zamek Ujazdowski) is a castle in the historic Ujazdów district, between Ujazdów Park (Park Ujazdowski) and the Royal Baths Park (Łazienki Królewskie), in Warsaw, Poland.

Its beginnings date to the 13th century, and it was rebuilt several times. Like many structures in Warsaw, it sustained much damage in the Warsaw Uprising (1944). Reconstructed 30 years later (1974), it now houses Warsaw's Center for Contemporary Art.

The first castle on the spot was erected by the Dukes of Masovia as early as the 13th century. However, in the following century their court was moved to the future Royal Castle in Warsaw, and the Ujazdów Castle fell into neglect. In the 16th century, a wooden manor was built there for Queen Bona Sforza. It was at Ujazdów Castle, on January 12, 1578, that Jan Kochanowski's blank-verse tragedy The Dismissal of the Greek Envoys received its premiere during the wedding of Jan Zamoyski and Krystyna Radziwiłł.

The ruins of the castle of the Mazovian princes were then incorporated into a new fortified manor built by King Sigismund III Vasa for his son, future King Władysław IV Vasa. However, there is little evidence that the residence was ever used by the young prince, who spent much of his youth either at his father's court. According to Adam Jarzębski's "Short Description of Warsaw" from 1643, the entrance to the Ujazdów Castle was adorned with four marble lions ("I lwy cztery generalne, Między nimi, naturalne, Właśnie żywe wyrobione, A z marmuru są zrobione; Nie odlewane to rzeczy, Mistrzowską robotą grzeczy"). Between 1659 and 1665, the building housed the mint of Titus Livius Boratini, who there struck his famous boratynka, a type of copper coin.

Again neglected, in 1674 the castle was bought by Stanisław Herakliusz Lubomirski and then rented to King Augustus II, who ordered the construction there of a new royal residence. The castle, incorporating much of the earlier constructions on the site, was built by Tylman of Gameren, a notable 17th-century architect and engineer. The gardens surrounding the castle, later divided into two separate parks, were refurbished. About that time the Łazienki Eremity and Łazienki Palace were built.

The castle's design was further modified by King Stanisław II August, who in 1764 commissioned Jakub Fontana, Dominik Merlini, Jean-Baptiste Pillement and Efraim Schroeger to refurbish it. The eastern and western façades were made taller by the addition of a second floor, while the post-Gameren outbuildings were also rebuilt to the height of the main building, thus creating a large courtyard. About that time, the castle was also included in the so-called Stanislavian Axis, a line of parks and palaces planned in the southern outskirts of Warsaw much like the Saxon Axis in the city center. The palace's reconstruction was almost complete by 1784, when work was abandoned and the building donated to the Polish Army.

Consequently, between 1784 and 1789 the castle was yet again rebuilt, this time by Stanisław Zawadzki, who converted it into military barracks. The outbuildings were enlarged substantially. Since that time the building housed the Lithuanian Foot Guard Regiment and the 10th Foot Regiment. During the Kościuszko Uprising the castle was the main centre of conscription for the 20th Foot Regiment. After the Partitions of Poland, during the Prussian occupation of Warsaw, the building was abandoned. After the proclamation of the Duchy of Warsaw it was again restored to the army and was converted into a military hospital. However, the plans of converting it to the Central Military Hospital of the Polish Army were postponed by the Congress of Vienna which awarded the Congress Poland to Russia. On April 1, 1818 the hospital was officially opened. It had places for up to 1000 wounded soldiers. After the outbreak of the November Uprising the hospital was enlarged to 1250 beds and an additional annex with places for 600 was opened in the nearby Łazienki complex.

After the fall of the uprising, the Russian garrison of Warsaw was significantly strengthened, while the Polish military units were disbanded. A new central military hospital was built next to what became the Ujazdów Park and the castle became more of a barrack for the Russian military personnel. Around 1850 the outbuildings were again rebuilt by Jerzy Karol Völck, but were partially demolished 20 years afterwards. After the outbreak of World War I the building was again converted to a provisional hospital by the Russians. Captured by the German army in 1915, on April 10, 1917 it was transferred to the Polish Legions and became the main military hospital for Polish units fighting alongside the Central Powers (the more modern Ujazdów hospital located nearby remained a German-only hospital).