Theodor S. Slen (October 15, 1885 – July 4, 1986) was an American educator, lawyer, and politician.
Slen was born in Delavan, Faribault County, Minnesota.
He lived with his wife and family in Madison, Lac qui Parle County, Minnesota and practiced law.
He served in the Minnesota National Guard during the Mexican Border Conflict and World War I. Slen served as a probate judge and attorney for Madison, Minnesota and as the Lac qui Parle County Attorney.
Slen served in the Minnesota House of Representatives from 1935 to 1940 and was an Independent and was a Democrat.