An alcázar (from Arabic: القصر, romanized: al-qaṣr, lit. 'the fort'; see below) is a type of Islamic castle or palace in Spain built during Muslim rule between the 8th and 15th centuries. They functioned as homes and regional capitals for governmental figures throughout the Umayyad caliphate and later for Christian rulers following the Iberian Reconquista. The term alcázar is also used for many medieval castles built by Christians on earlier Roman, Visigothic or Islamic fortifications and is frequently used as a synonym for castillo or castle. In Latin America there are also several colonial palaces called alcázars.

The Spanish word alcázar (pronounced [alˈkaθaɾ]) derives from the Arabic word القصر al-qaṣr 'the fort/castle/palace', that in turn derives from the Latin word castrum ('fortress', 'military camp').

Similar words exist in Galician (alcázar, pronounced [alˈkaθɐɾ]), Portuguese (alcácer, pronounced [ɐlˈkasɛɾ]), and Catalan (alcàsser, pronounced [əlˈkasəɾ]).

Spain also has Muslim citadels known as alcazabas (القصبة al-qaṣbah). However, not all castles in Spain are called alcázars: the majority are called castillo in Spanish or castell in Catalan. Nor was every alcázar or alcazaba in Iberia built by the Muslims: many castles with these names were built after the Islamic caliphate was expelled from the Iberian Peninsula. Following the Spanish Reconquista, Christian patrons built or refurbished palaces to resemble the Islamic style, known as mudéjar.

Often, alcázars are described as "Moorish", a term used by Europeans to refer to followers of the Islamic faith. While "Moors" has been used for centuries to reference Spanish Muslims, today however, the discussion of "Moorishness" is often approached with care. The term has been radicalized historically in Spain to signify the Other and exoticize the Muslim population. Modern language utilizes the terms Islamic, Muslim, and Andalusi instead of Moorish, but it is worth clarifying the historical context.

Alcázars were owned by Islamic, and later, Christian rulers of al-Andalus, the buildings symbolized the ruler's power and wealth over the region. Over time, the different cultural influences of the buildings merged, marking the alcázar as a transmission among Islamic, Christian and Jewish cultures.

Alcázars first became constructed under the Umayyad dynasty. It was after conquering the Visigothic Kingdom when the Umayyads began expanding on their architecture to create a sophisticated Islamic empire. After the 717 conquest, Umayyad caliphs established their capital in Córdoba, Spain, transforming the former Visigoth palace into the Alcázar of the Caliphs.

The reign of the Umayyad dynasty in Arabia ended with the rise of the Abbasid caliphate in the second half of the eighth century. Umayyad caliph Abd al-Rahman I fled the Middle East, traveling west for five years before finally settling in what would become known as al-Andalus after the Islamic conquest of Iberia in 711. He and his descendants built palaces equivalent to grand cities that became homes to rulers and Andalusian elites.