Madeira Malvasia (or Malmsey) is a kind of fortified wine made from the Malvasia family of grape varieties under Portugal's Madeira DOC. It is the sweetest, richest form of Madeira, and is typically made in a style that contains more than twice the residual sugar found in Madeira Sercial or Madeira Verdelho. The next sweetest in the spectrum is Madeira Bual.

The name Malvasia has come to represent several different grape varieties around the Mediterranean. This name is thought to be derived from Monemvasia, the name of a bustling Greek port which, like Madeira, was a natural waypoint on trading routes and became strongly associated with the sweet, rich wine styles most commonly found there.

Malmsey is a second corruption of the name, first uttered by the twisted tongues of 18th-Century British mariners. It came to represent this rich, sweet style of wine made around the Mediterranean. It now refers specifically to the wines made from Malvasia in Madeira.

The Malvasia grapes that produce Malmsey are typically grown in warm coastal locations at low altitude, particularly at Câmara de Lobos, This is a peaceful fishing village, though its name means "the chamber of wolves". The grape variety retains its acidity as it develops sugar, and as a consequence, Madeira Malvasia wines are some of the most long lived in the world.

Confusingly the Boal/Bual grape is also known as Malvasia Fina. But it is distinct from the group of varieties used for Malmsey.

Malmsey's greatest claim to fame comes from the quill of William Shakespeare. His play Richard III sees George Plantagenet, the Duke of Clarence, drowned in the "malmsey-butt". Clarence's ghost later returns to haunt King Richard, moaning that he was "wash'd to death with fulsome wine".