The Under Secretary of Defense (Comptroller)/Chief Financial Officer, abbreviated USD(C)/CFO, is a high level civilian official in the United States Department of Defense. The Under Secretary of Defense (Comptroller) is the principal staff assistant and adviser to both the Secretary of Defense and the Deputy Secretary of Defense for all budgetary and fiscal matters, including the development and execution of the Defense Department's annual budget.

The United States Department of Defense is an executive branch department of the federal government of the United States charged with coordinating and supervising all agencies and functions of the U.S. government directly related to national security and the United States Armed Forces. As of June 2022, the U.S. Department of Defense is the largest employer in the world, with over 1.34 million active-duty service members, including soldiers, marines, sailors, airmen, and guardians. The Department of Defense also maintains over 778,000 National Guard and reservists, and over 747,000 civilians bringing the total to over 2.87 million employees. Headquartered at the Pentagon in Arlington County, Virginia, just outside Washington, D.C., the Department of Defense's stated mission is to provide "the military forces needed to deter war and ensure our nation's security".