McCain, a strong supporter of the U.S. military mission in Iraq, is believed to be staying in the country for about 24 hours.
"Senator McCain is in Iraq and will be meeting with Iraqi and U.S. officials," said Mirembe Nantongo, spokesperson for the U.S. Embassy in Baghdad.
This is his eighth visit to Iraq. He's accompanied by Sens. Joe Lieberman, I-Conn., and Lindsey Graham, R-S.C. Before leaving McCain said his trip to the Middle East and Europe was a fact-finding venture, not a campaign photo opportunity.
The senator last met with Iraq Prime Minister Nouri al-Maliki last November during the Thanksgiving holiday.
McCain was to meet Sunday with Iraqi Deputy Prime Minister Barham Saleh. Later in the day, he and Gen. David Petraeus, the top commander in Iraq, were planning to talk. It was also thought McCain would meet with al-Maliki.
"We were informed that John McCain landed in Iraq Sunday morning. A meeting will take place with the Iraqi government," said Ali al-Moussawi, an official in the Iraqi prime minister's office.
There were no details immediately available about McCain's meetings and his schedule for the day apparently remained in flux, a U.S. official said on condition of anonymity as the official was not authorized to release the information.
McCain's weeklong trip also includes stops in Israel, Jordan, Britain and France.
McCain is expected to meet with British Prime Minister Gordon Brown for the first time, and French President Nicolas Sarkozy for the third time. He met and corresponded with Sarkozy both before and after the French president was elected. The two last saw each other last summer.
McCain has relationships with every leader in Israel he plans to see, including Prime Minister Ehud Olmert, Foreign Minister Tzipi Livni, Defense Minister Ehud Barak and hawkish opposition leader Benjamin Netanyahu.
Ahead of the trip, McCain expressed worries that insurgents might try to influence the November presidential election with increased attacks in Iraq.
"Yes, I worry about it," he said Friday in Springfield, Pennsylvania. "And I know they pay attention, because of the intercepts we have of their communications."
McCain told reporters later that al-Qaida remains smart and adaptable despite an increase of U.S. troops in Iraq.
A defiant supporter of the 2003 invasion and President Bush's troop increase last year, McCain is likely to focus in Iraq on the drop in sectarian violence and U.S. and civilian casualties since last summer.