Coffman left a legislative seat in the early 90's to fight in the Gulf War, the Denver Post reported. He plans to get married May 28 and report to duty in June.
"I've been pulled in two different directions," he said. "I love my work here and it has been such an honor to be the state treasurer, but in my heart I know that my skills and training are needed in Iraq."
Coffman will hold the rank of major in a civil affairs unit.
The Republican told Gov. Bill Owens that he plans to return as state treasurer and fill out the remainder of his four-year term.