Unterwellenborn is a municipality located in the district of Saalfeld-Rudolstadt within the state of Thuringia, Germany. The current administrative boundaries were established on February 1, 2006, through the merger of several surrounding areas including Birkigt, Goßwitz, Könitz, Lausnitz bei Pößneck, and the original municipality of Unterwellenborn.
The municipality expanded further in the 1990s when Dorfkulm, Langenschade, and Oberwellenborn were incorporated into its territory on April 9, 1994. A subsequent merger occurred in July 2018, which integrated the former municipality of Kamsdorf into Unterwellenborn.