DeKalb County was established on December 2, 1837 by an act of the General Assembly of the State of Tennessee. The County was formed from parts of Smith, Warren, White, and Cannon Counties. The county was named by the Legislature. It was named in honor of Johann DeKalb, who was born in Germany and served as a Major General during the American Revolution.

The Legislature also specified that the County seat should be named Smithville, in honor of Samuel Granville Smith. He had served as Mayor of Gainesboro, As a State Senator and was Tennessee's Secretary of State at the time of his death.

The First settler of DeKalb County was Adam Dale who was born in 1768 in Worchester County on the eastern shore of Maryland. The earliest town in DeKalb County was Liberty. It was founded in 1804 by Adam Dale and his Mill was the first business to be established.

The founder of Alexandria, DeKalb's second oldest town was Daniel Alexander, who named the town for himself. The only other incorporated town in the County is Dowelltown. It was not incoporated until 1949.

DeKalb County sits on two of the geological regions found in Tennessee. The Highland Rim and the Central Basin. It consists of three main areas: the Smith Fork area, the Caney Fork area, and the Highland Rim Plateau, locally known as "the flatwoods."

What is now DeKalb County had been a part of Smith, Warren, White and Cannon Counties. Alexandria, Liberty, and Temperance Hall were in Smith County. The line between Smith and Warren Counties cut across Holmes creek, Snows Hill, and Dry Creek. Most of "the flatwoods" area and north and east to the Caney Fork was in Warren County. East of the Caney Fork from Little Hurricane Creek to Rock Island was in White County. In 1854, a large section on the northeast corner was given to Putnam County.

· Incorporated Towns in Dekalb County: Smithville, Alexandria, Liberty, and Dowelltown
· Unincorporated Communities: Allens Chapel, Antioch, Bethel, Belk, Blue Springs, Bluhmtown, Buckners Chapel, Cherry Hill, Cross Roads, Dismal, Dry Creek, Helton, Indian Mound, Jefferson, Keltonburg, Laurel Hill, Midway, Mt. Moriah, Pisgah, Phillipi, Shiney Rock, Snows Hill, Temperance Hall, Watkins