Commonly called a unit crest or DUI for short, the Distinctive Unit Insignia worn by personnel in the 109th Regional Support Group was originally approved for the 109th Engineer Group on 5 May 1971, but on 1 September 2008 was redesignated for the 109th Support Group with updated description and symbolism.
Scarlet and white hues in the insignia are the colors used for the Corps of Engineers, the original designation of the unit. At the top of the crest is a triple-towered wall rising above the nonagon (nine-sided) disc in the center; this is a reference to the old city walls of Manila and denotes campaign participation in the Philippine Insurrection. Bookending the wall are a pair of fleurs-de-lis; these represent service in both World War I (six campaigns) and World War II (seven campaigns). Inside the nonagon, two crosses with discs in their centers represent two French Croix de Guerres awarded to the unit.
Between and above the crosses is a gold, radiated disc: this is a depiction of the sun from the flag of South Dakota, the unit’s home state. At the base of the nonagon are a series of black peaks, an obvious reference to the famed Black Hills of South Dakota. The vertical, white wavy stripe simulates Rapid City, the headquarters for the Group. “First To Serve” is likely a reference to the unit’s being the first South Dakota National Guard unit ever to be mustered into Federal service.
Full guidance on wear of the DUI is found in DA Pamphlet 670-1, Section 21-22, "Distinctive unit insignia" and 21–3(d) and (e), "Beret" and "Garrison Cap," respectively.As of Summer 2023, the 109th Regional Support Group remains an active unit in the South Dakota Army National Guard and is still headquartered at Rapid City, South Dakota.