Bolinus brandaris (originally called Murex brandaris by Linnaeus); and commonly known as the purple dye murex or the spiny dye-murex; is a species of medium-sized predatory sea snail; an edible marine gastropod mollusk in the family Muricidae; the murex snails or the rock snails.

Tyrian purple (Greek; p??f??a; porphyra; Latin: purpura); also known as royal purple; imperial purple or imperial dye; is a purple-red natural dye; which is a secretion produced by certain species of predatory sea snails in the family Muricidae; a type of rock snail by the name Murex. This dye was probably first used by the ancient Phoenicians. The dye was greatly prized in antiquity because the color did not easily fade; but instead became brighter with weathering and sunlight.

Tyrian purple was expensive: the 4th-century-BC historian Theopompus reported; 'Purple for dyes fetched its weight in silver at Colophon' in Asia Minor. The expense meant that purple-dyed textiles became status symbols; and early sumptuary laws restricted their uses. The production of Tyrian purple was tightly controlled in Byzantium and was subsidized by the imperial court; which restricted its use for the colouring of imperial silks; so that a child born to a reigning emperor was styled 'porphyrogenitos' or 'born in the purple'.