Bill Buchanan is a fictional character in 24. Played by James Morrison, the character is the head of the Los Angeles Counter Terrorist Unit (CTU) from the fourth season to the sixth season, when he is forced to step down. In between Season 5 and 6, he married Karen Hayes. He returns in Season 7, working outside of official agencies to uncover a terrorist plot that goes to the highest levels of government.

Buchanan has a Bachelor of Arts in English from Brown University. Prior to becoming the Special Agent in Charge at the LA CTU office, he was the Associate Special Agent in Charge at the Los Angeles Division office. Buchanan started his career as an agent with the New York Domestic Unit.