Major General Duncan Edward Lewis AO, DSC, CSC (born 3 August 1953) is an Australian military officer, diplomat and intelligence chief. From 2014 to 2019, he was the Director-General of Security of the Australian Security Intelligence Organisation (ASIO). Prior to that appointment, he held the post of Australian Ambassador to the Kingdom of Belgium, the Grand Duchy of Luxembourg, the European Union and the North Atlantic Treaty Organization. He is a retired Australian Army officer, and formerly Special Operations Commander Australia (2002–2004), National Security Adviser, and Secretary of the Department of Defence.