He touched down in the southern city of Basra, where many British troops are based.
Tight security surrounded Mr Blair's fourth visit to Iraq since the March 2003 invasion.
A total media blackout was in place until he touched down from Kuwait, for fear of attacks by insurgents.
He is due to meet with the top US military commander in Iraq, General George Casey, and US ambassador Zalmay Khalilzad, along with senior British diplomatic and military figures to discuss security, politics and the presence of foreign troops, his official spokesman said.
A spokesman said the trip would partly be to gain an assessment of the general election situation, partly to get a military assessment of the security situation and also to share with Americans "our view of the way forward."
When asked to elaborate, the spokesman highlighted the need for the results of the December 15 election to be announced and the subsequent formation of a new government, as well as Iraqi security forces' ability to take control of security issues.
Mr Blair's trip comes at a critical time for war-ravaged Iraq, which has been struggling to re-establish itself after the toppling of former dictator Saddam Hussein.
Iraqis are waiting to hear the outcome of their first polls for a full-term government since the ousting of Saddam's Baathist regime.
London has hinted it hopes to start pulling out its military contingent from 2006 once local security forces are up to speed, and the election is seen as a key element in securing this exit.
The British prime minister also wants to say thank you to Britain's 8,000 or so troops deployed largely in Iraq's four southern provinces for serving away from home, especially over Christmas.
However he is not expected to meet with any Iraqi soldiers, many of whom have been trained by their British counterparts.
With the election results not yet out, Mr Blair is also not scheduled to hold talks with any of Iraq's political figures.
Blair last visited Iraq on December 21 2004 when he first visited Baghdad.
He also travelled just to Basra in January 2004 and May 2003.