Sir Jonathan Evans is to step down as head of MI5 next month, Home Secretary Theresa May has told MPs.
Mrs May paid tribute to Sir Jonathan's 33 years of work with the Security Service.
He became the Director General of MI5 in April 2007, when he succeeded Lady Manningham-Buller.
His departure comes at the end of his contract term.
Speaking during questions for her department in the Commons, the Home Secretary said: "During this time his contributions have varied from investigating counter espionage, developing and implementing key policies on security and most recently countering the threat of international terrorism."