Augustine Thomas O'Donnell, Baron O'Donnell, is a former British senior civil servant and economist, who between 2005 and 2011 served as the Cabinet Secretary, the highest official in the British Civil Service.

The Cabinet Secretary is the most senior civil servant in the United Kingdom and is based in the Cabinet Office. The person in this role acts as the senior policy adviser to the Prime Minister and Cabinet, and as the Secretary to the Cabinet is responsible to all ministers for the efficient running of government. The role is currently occupied by Simon Case.