She flew in from Bahrain under tight security on a C-17 military transport place with a full entourage of officials and press, in what is her second visit to the war-battered country as chief US diplomat.
Her visit to Iraq came a day after a suicide bomber blew himself up in a Baghdad restaurant, killing more than 30 people.
Ms Rice's visit was kept under wraps until she landed in Mosul, 500 kilometres north of Baghdad, where she will meet with local leaders and US military and civilian officials.
She told reporters her main aim is to encourage Iraq's fractious political and religious groups to bridge their differences before next month's general elections, into a single, united country where everyone feels "fully protected".
The visit came five weeks before elections for a permanent parliament for Iraq that will cap efforts to restore self-rule after the March 2003 invasion to oust Saddam Hussein.
She said the US will not throw its support behind any candidate in particular.
"I want to talk about the importance of reaching across the sectarian divide, and the future of Iraq has to be one which includes everyone," said Ms Rice.
Faced with a raging insurgency that shows no sign of abating, she called on Iraq's majority Shi'ite Muslims, Kurds and minority Sunnis to end their squabbling and work to unite the country.
"We do urge all parties and all potential candidates to reach out across sectarian lines to deal with any divisions, to try and have programs that make for a single Iraq," Ms Rice told reporters accompanying her.
During her tour she will launch an initiative aimed at promoting democratic and economic reform in the country.
Ms Rice is to take her message back to Bahrain where she will meet Arab officials at a conference on regional democracy, and will also visit Saudi Arabia where she will hold strategic talks with a close US ally.
She said Iraq must be integrated fully into the Arab world and "obviously we want as much Sunni participation as possible in these next elections."
"The Saudis have a lot of contacts, tribal and other contacts, that I would hope they would use and would press the Sunnis to be involved in a constructive way," she said.
Washington fears that marginalisation of the Sunnis from the political process could fuel the insurgency and delay prospects for the withdrawal of some 138,000 American troops still in Iraq.
Mosul, one of the largest and most ethnically mixed cities in Iraq, is 50 percent Sunni and was a flashpoint of the insurgency until a concerted cleanout effort seven months ago, US officials said.