ShieldPer bend indented Argent and Gules a prickly pear cactus Proper between in chief a castle of the like (from the Spanish Campaign Medal), and in base a fleur-de-lis of the first.
CrestThat for the regiments and separate battalions of the South Dakota Army National Guard: From a wreath Argent and Gules, a coyote statant Proper.
MottoALLONS Y FAIRE (Let’s Go Do It).
ShieldThe shield is white and red, the colors associated with the Engineer Corps, divided diagonally by a saw tooth partition line to signify combat Engineers. The castle is taken from the Spanish Campaign Medal and represents service during the Philippine Insurrection, the cactus is for Mexican Border duty, and the fleur-de-lis symbolizes World War service.
CrestThe crest is that of the South Dakota Army National Guard.
The coat of arms was originally approved for the 109th Engineer Regiment on 23 May 1927. It was redesignated for the 109th Engineer Battalion on 2 September 1943. It was rescinded (cancelled) on 22 December 1969. The insignia was reinstated for the 109th Engineer Battalion on 13 January 1999.