Karl Friedrich August Kahnis (22 December 1814 - 20 June 1888) was a German Lutheran theologian.

He was born at Greiz, studied at Halle, and in 1850 was appointed professor ordinarius at Leipzig. Ten years later he was made canon of Meissen. He retired in 1886, and died in 1888 at Leipzig.

Kahnis was at first a neo-Lutheran, blessed by E. W. Hengstenberg and his pietistic friends. He then attached himself to the Old Lutheran party, interpreting Lutheranism in a broad and liberal spirit and showing some appreciation of rationalism. His Lutherische Dogmatik, historisch-genetisch dargestettt (3 vols., 1861-1868; 2nd ed. in 2 vols., 1874-1875), by making concessions to modern criticism, by spiritualizing and adapting the old dogmas, by attacking the idea of an infallible canon of Scripture and the conventional theory of inspiration, by laying stress on the human side of Scripture and insisting on the progressive character of revelation, brought him into conflict with his former friends. A. W. Diekhoff , Franz Delitzsch (Fur und wider Kahnis, 1863) and Hengstenberg (Evangelische Kirchenzeitung, 1862) protested loudly against the heresy, and Kahnis replied to Hengstenberg in a vigorous pamphlet, Zeugniss fiir die Grundwahrheiten des Protestantismus gegen Dr Hengslenberg (1862).

Other works by Kahnis are Lehre vom Abendmahl (1851), Der innere Gang des deutschen Protestantismus sett Mitte des vorigen Jahrhunderts (1854; 3rd ed. in 2 vols., 1874; Eng. trans., 1856); Christentum und Luthertum (1871); Geschichte der deutschen Reformation, vol. i. (1872); Der Gang der Kirche in Lebensbildern (1881, &c.); and Uber das Verhaltnis der alien Philosophie zum Christentum (1884).