Stephen John Hadley (born February 13, 1947, in Toledo, Ohio) was the 21st U.S. Assistant to the President for National Security Affairs (commonly referred as National Security Advisor), serving under President George W. Bush. He had been Assistant to the President and Deputy National Security Advisor from January 22, 2001. On January 26, 2005, he replaced Condoleezza Rice as National Security Advisor, upon Rice's confirmation as Secretary of State.

Hadley served as a senior foreign and defense policy advisor to then-Governor Bush during the 2000 presidential campaign and worked in the Bush-Cheney Transition on the National Security Council.

Previous to this position, he was a partner in the Washington, D.C. law firm of Shea & Gardner and a principal in The Scowcroft Group, Inc., an international consulting firm.