Stephan Gerlach (Latin: Stephanus Gerlachius) was a German Lutheran theologian.

He was tasked with a special mission in Constantinople, namely to establish an alliance between Orthodoxy and Lutheranism against Catholicism.

From 1573 to 1578 he was in Constantinople as the first assistant and clergyman of the ambassador of the Holy Roman Empire - Baron David Ungnad von Sonnegg.

He wrote a diary, one of the most cited sources in the historical literature about the situation in Constantinople in recent years of the "Kingdom of the Slavs", i.e. after the death of Selim II and just before the outbreak of the Long Turkish War.

Upon his return, he became professor of theology (1586) and head of the Protestant district in Tübingen, and he wrote against Calvinists and Jesuits.