Zellij (Arabic: زليج, romanized: zillīj), also spelled zillij or zellige, is a style of mosaic tilework made from individually hand-chiseled tile pieces. The pieces were typically of different colours and fitted together to form various patterns on the basis of tessellations, most notably elaborate Islamic geometric motifs such as radiating star patterns composed of various polygons. This form of Islamic art is one of the main characteristics of architecture in the western Islamic world. It is found in the architecture of Morocco, the architecture of Algeria, early Islamic sites in Tunisia, and in the historic monuments of al-Andalus (in the Iberian Peninsula). From the 14th century onwards, zellij became a standard decorative element along lower walls, in fountains and pools, on minarets, and for the paving of floors.

After the 15th century, the traditional mosaic zellij fell out of fashion in most countries except for Morocco, where it continues to be produced today. Zellij is found in modern buildings making use of traditional designs such as the Hassan II Mosque in Casablanca which adds a new color palette with traditional designs. The influence of zellij patterns was also evident in Spanish tiles produced during the Renaissance period and is seen in some modern imitations painted on square tiles.

The word zillīj (زليج) is derived from the verb zalaja (زَلَجَ) meaning "to slide," in reference to the smooth, glazed surface of the tiles. The word azulejo in Portuguese and Spanish, referring to a style of painted tile in Portugal and Spain, derives from the word zillīj. In Spain, the mosaic tile technique used in historical Islamic monuments like the Alhambra is also referred to as alicatado, a Spanish word deriving from the Arabic verb qata'a (ﻗَﻄَﻊَ) meaning "to cut".

The technique of mosaic tilework was probably inspired or derived from Byzantine mosaics and then adapted by Muslim craftsmen for faience tiles. Fragments of zellij tilework from al-Mansuriyya (Sabra) in Tunisia, possibly dating from either the mid-10th century Fatimid foundation or from the mid-11th Zirid occupation, suggest that the technique may have developed in the western Islamic world around this period. Georges Marçais argued that these fragments, along with similar decoration found at Mahdia, indicate that the technique likely originated in Ifriqiya and was subsequently exported further west.

By the 11th century, the zellij technique had reached a sophisticated level in the western Islamic world, as attested in the elaborate pavements found at the Hammadid capital, Qal'at Bani Hammad, in Algeria. Modern excavations here have uncovered a series of interlocking tiles including cross-shaped lustre-painted tiles and eight-pointed star-shaped tiles of monochrome green or turquoise. Found in palaces built between 1068 and 1091, these might be attributable to Ifriqiyan craftsmen who fled the Banu Hilal invasions to the east and sought refuge with the Hammadids around this time, though the lustre tiles may have been imported from elsewhere. In general, the Hammadid palaces made greater use of glazed ceramic architectural decoration than earlier Islamic architecture and may have played a role in promoting this architectural fashion to the rest of the western Islamic lands.

During the subsequent Almohad period, prominent bands of ceramic decoration in green and white were features on the minarets of the Kutubiyya Mosque and the Kasbah Mosque of Marrakesh. Relatively simple in design, they may have reflected artistic influences from Sanhaja Berber culture. Jonathan Bloom cites the glazed tiles on the minaret of the Kutubiyya Mosque, dating from the mid-12th century, as the earliest reliably-dated example of zellij in Morocco. The individual tile pieces are large, allowing the pattern to be visible from afar. Each piece was pierced with a small hole prior to being baked so that the tiles could be affixed by nails to a wooden frame set into a mortar surface on this part of the minaret. The minaret of the Kasbah Mosque, built slightly later in the 1190s, makes greater use of ceramic decoration generally, including geometric mosaics on the upper parts of the minaret in the same technique as those of the Kutubiyya (though slightly more varied in design). The tiles on both minarets today are modern reproductions of the originals, of which fragments have been preserved in the reserve collection of the Badi Palace museum.

The more complex zellij style that we know today became widespread during the first half of the 14th century under the Marinid, Zayyanid, and Nasrid dynastic periods in Morocco, Algeria, and al-Andalus. Due to the significant cultural unity and relations between al-Andalus and the western Maghreb, the forms of zellij under Marinid, Nasrid, and Zayyanid patronage are extremely similar. In Ifriqiya (Tunisia), under the Hafsid dynasty, zellij tiling largely fell out of style during this same period and was replaced by a preference for stone and marble paneling.

Zellij tiling was most typically used to pave floors and to cover the lower walls inside buildings. Zellij was also used on the exterior of minarets and on some entrance portals. Geometric motifs predominated, with patterns of increasing complexity being formed during this period. Less frequently, vegetal or floral arabesque motifs were also created. On walls, zellij geometric dadoes were commonly topped by an epigraphic frieze. By this period, more colours were employed such as yellow (using iron oxides or chrome yellow), blues, and a dark brown manganese colour. This style of tile mosaic, formed by assembling a large number of small hand-cut pieces to form a pattern, is evident in famous buildings of the period such as the Alhambra palaces of the Nasrids, the mosques of Tlemcen, and the Marinid madrasas of Fez, Meknes, and Salé. It is also found in some Christian Spanish palaces of the same period who employed Muslim or Mudéjar craftsmen, most notably the Alcazar of Seville, whose 14th-century sections are contemporary with the Alhambra and contain zellij tilework in the same style, although of slightly lesser sophistication.