Adriaen Thomasz. Key, was a Flemish painter of portraits and religious paintings, a draughtsman and a printmaker. He worked for a while in the Antwerp workshop of the prominent history and portrait painter Willem Key and later took over the workshop. His work was highly regarded in his time for its technical mastery and innovative concepts and thus influenced a later generation of painters including Rubens.

Very little is known about the origins and early life of Adriaen Thomasz. Key. Research has demonstrated that contrary to what was believed traditionally, his family name was not Key and he was no relation of Willem Key. He only adopted the Key family name after taking over the workshop of Willem Key in 1567. Before that time he only used the monogram AT and the artist was referred to solely as Adriaen Thomasz. (meaning Adriaen, son of Thomas). It is clear that the reason why he adopted the family name is that Willem Key and his workshop enjoyed a high reputation and using Key as his family name was a good branding tool for his own work.

He was likely born in Antwerp around 1544. His father's first name was Thomas. He trained initially under the glass painter Jan Hack III and was registered as a pupil at the Antwerp Guild of Saint Luke in the guild year 1558. He became linked to the workshop of Willem Key at an early age and Willem Key is regarded as another master of Adriaen.

The Habsburg Netherlands were at the time affected by the Beeldenstorm (Iconoclastic Fury) that commenced in the early 1560s and reached its peak in 1566. During the period of iconoclasm, Catholic art and many forms of church fittings and decoration were destroyed by nominally Calvinist Protestant crowds who supported the Protestant Reformation. Antwerp was also affected by the wave of destruction. Key was able to emerge as one of the leading painters in Antwerp in the second half of the sixteenth century after the iconoclasm had abated. He was admitted as a master painter in the Guild in 1567.

When Willem Key died in 1568, Adriaen Thomasz took over his workshop. Although he had converted to Calvinism he did not leave Antwerp after the Fall of Antwerp in 1585 when the Calvinist government of Antwerp was overthrown by Spanish forces led by Alessandro Farnese. He received pupils in his workshop in 1582, 1588 and 1589. He received commissions from the leader of the Dutch Revolt William of Orange and the socio-economic elite of Antwerp. He is last mentioned in a document in 1589. It is not clear whether he died soon after or left Antwerp because of his Calvinist opinions.