Caspar Aquila (sometimes Kaspar or Gaspar Aquila; 7 August 1488 – 12 November 1560), born Johann Kaspar Adler, was a German Lutheran theologian and reformer.
He was born at Augsburg, and educated there, at Ulm (1502), in Italy (he met Erasmus in Rome), at Bern (1508), and studied theology in Leipzig (1510) and Wittenberg (1513). According to his son, he entered the ministry in August 1514, while at Bern. He was for some time a military chaplain.
In 1516, he became pastor of Jengen, near Augsburg, where he introduced ideas of the Reformation. Openly proclaiming his adhesion to Martin Luther's doctrine, he was imprisoned for half a year (1520 or 1522) at Dillingen, by order of the bishop of Augsburg; a death sentence was commuted to banishment through the influence of Isabella, wife of Christian II of Denmark and sister of Charles V. Returning to Wittenberg, he met Luther, and acted as tutor to the sons of Franz von Sickingen at Ebernburg castle. After the siege of the Ebernburg by Richard Greiffenklau, the archbishop of Trier, on 6 June 1523, he returned to Wittenberg to teach Hebrew, and aided Luther in his version of the Old Testament.