Theobald Otjen (October 27, 1851 – April 11, 1924) was a U.S. Representative from Wisconsin.

Theobald was born to German immigrants John Conrad and Dorothea (Schreiner) Otjen, in west China Township in St. Clair County, Michigan and attended the Marine City (Michigan) Academy and a private school in Detroit.

He graduated from the law department of the University of Michigan at Ann Arbor on March 25, 1875.

He was admitted to the bar at Ann Arbor in 1875 and commenced practice in Detroit, Michigan.

Theobald married Louisa Elizabeth Heames, daughter of prominent Detroit civil engineer Henry Heames, in Detroit on March 27, 1879, and had five children between 1880 and 1892.