The Ghent Altarpiece or Adoration of the Mystic Lamb (Dutch: Het Lam Gods or The Lamb of God; completed 1432) is a very large and complex Early Flemish polyptych panel painting which is considered to be one of Belgium's masterpieces and one of the world's treasures.

It was once in the Joost Vijdt chapel at Saint Bavo Cathedral, Ghent, Belgium, but was later moved for security reasons to the chapel of the cathedral. Commissioned by the wealthy merchant and financier Joost Vijdt for his and his wife's private chapel, it was begun by Hubert van Eyck, who died in 1426 while work was underway, and completed by his younger brother Jan van Eyck. The altarpiece represented a "new conception of art", in which the idealization of the medieval tradition gave way to an exacting observation of nature.

The altarpiece consists of a total of 24 framed panels, which make up two views, open and closed, which are changed by moving the hinged outer wings. The upper register (row) of the opened view shows Christ the King (but see below) between the Virgin Mary and John the Baptist. The insides of the wings represent angels singing and making music, and on the outside Adam and Eve. The lower register of the central panel shows the adoration of the Lamb of God, with several groups in attendance and streaming in to worship, overseen by the dove representing the Holy Spirit. On weekdays the wings were closed, showing the Annunciation of Mary and donor portraits of Joost Vijdt and his wife Lysbette Borluut.

Hubert van Eyck (also Huybrecht van Eyck) (c. 1385–90 – 18 September 1426) was a Flemish painter and older brother of Jan van Eyck. He was probably born in Maaseik, Flanders, now in Belgium.

He became court painter to Philip the Good, Duke of Burgundy, settling in Ghent by c. 1420. Shortly afterwards, he began his only surviving documented work, the Ghent Altarpiece in the Saint Bavo Cathedral. However the painting was not finished until six years after his death, in 1432, so the degree to which the surviving altarpiece reflects his work, rather than that of Jan who took it over, remains much discussed. Writing in 1933, art historian Bryson Burroughs, who at that time attributed to Hubert the Crucifixion and Last Judgement diptych and whom he describes as "the fountainhead of northern painting", suggests he outlined the Ghent Altarpiece with Jan painting in after his brother's death. An inscription on the altarpiece, presumably composed by Jan, credits Hubert with the inspiration and major role in the work, but today this is often regarded as overgenerous. Given the circumstances, the Ghent Altarpiece is a difficult work to use for comparison when assessing other attributions, especially as several other artists from the brothers' workshops probably worked on it as well.