The Office of the Secretary of Defense (OSD) is a headquarters-level staff of the United States Department of Defense. It is the principal civilian staff element of the U.S. Secretary of Defense, and it assists the Secretary in carrying out authority, direction and control of the Department of Defense in the exercise of policy development, planning, resource management, fiscal, and program evaluation responsibilities. OSD (along with the Joint Staff) is the Secretary of Defense's support staff for managing the Department of Defense, and it corresponds to what the Executive Office of the President of the U.S. is to the U.S. president for managing the whole of the Executive branch of the federal government.

OSD includes the immediate offices of the Secretary (SECDEF) and the Deputy Secretary of Defense (DEPSECDEF), as well as the Under Secretary of Defense for Research and Engineering; Under Secretary of Defense for Acquisition and Sustainment; Under Secretary of Defense for Policy; Under Secretary of Defense (Comptroller); Under Secretary of Defense for Personnel and Readiness; and Under Secretary of Defense for Intelligence & Security. All of these positions are Presidential appointments which require U.S. Senate confirmation, as do each of their sole deputies.

Other positions include the Assistant Secretaries of Defense, Assistants to the Secretary of Defense, General Counsel, Director, Operational Test and Evaluation, Director of Administration and Management, and other staff offices that the Secretary establishes in order to assist in carrying out their assigned responsibilities.