It was Blair's fourth trip to Iraq since the U.S.-led invasion in March 2003.
Blair flew into the southern city of Al-Basrah from Kuwait on a trip that, for security reasons, was not announced in advance.
Blair was expected to be briefed by senior British and U.S. officials on the security situation in the country and on the outcome of parliamentary elections held on 15 December.
Britain has some 8,000 troops in Iraq. At home, Blair is facing calls to pull out troops. British officials have not, however, publicly discuss a timetable for withdrawal.