The White House issued a statement saying President Barack Obama had received Fields' resignation and expressing appreciation for his work in Afghanistan and his "more than 40 years" securing the national defense.
"His team has helped lead the effort to provide comprehensive and independent oversight of fiscal initiatives in Afghanistan," the White House statement said. "Under General Fields' tenure, SIGAR produced numerous critical reports that have improved reconstruction efforts, and helped insure that U.S.-funded programs are achieving their objectives. General Fields' hard work and steadfast determination have established SIGAR as a critical oversight agency.
"As he moves on to new challenges, he can do so confident in the knowledge that the president and the American people owe him a debt of gratitude for his courage, leadership, and selfless service to our nation."
ABC News cited a White House source who said Fields was not forced out by the administration, though he has been heavily criticized in some quarters.
"He told us that since the administration is transitioning to a new phase in Afghanistan, he thought it was a good time to step down," ABC quoted the source as saying.
A bipartisan group of senators urged Obama last fall to remove Fields, a holdover from the George W. Bush administration, saying the special inspector general's office was a "failing organization."
The office is responsible for monitoring $56 billion the United States has allocated to Afghanistan since 2002 for humanitarian and non-military development programs.