Jack Brennan (born as Gottlieb Doering) was an American professional baseball catcher in the late 19th century.

In his five-year career he played with the St. Louis Maroons, Kansas City Cowboys of the American Association, Philadelphia Athletics of the AA, and the Cleveland Infants.

Brennan was born Gottlieb Doering in St. Louis, Missouri in 1862.

After his playing career ended in 1890, he continued to work as an umpire in the St. Louis Area.

This biographical article relating to a United States baseball catcher born in the 1860s is a stub.