right|thumb|
Johann Baptist Alzog (8 June 1808 – 1 March 1878) was a German theologian and Catholic church historian.
He was born at Ohlau, in Silesia. He studied at the universities of Breslau and Bonn and was ordained a priest at Cologne in 1834.
In the following year he accepted the chairs of exegesis and church history at the seminary of Posen. He held that post until his death at Freiburg.
Together with Ignaz von Döllinger, Alzog was instrumental in convoking the famous Munich assembly of Catholic scholars in 1863.