Four of the soldiers died at the scene and the fifth died later from wounds, the military said in a statement. The blast also wounded three U.S. soldiers and an Iraqi interpreter, the military said.
The statement said the soldiers were killed "when their dismounted patrol was struck by an explosion" and "initial reports indicated the explosive device was a suicide vest."
The name of the soldiers were being withheld pending notification of next of kin.
It was the deadliest attack since Jan. 28, when five U.S. soldiers were killed in a roadside bomb in the northern city of Mosul.
Monday's deaths brought the number to 3,979 members of the U.S. military who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.
In other developments:
Also Monday, a female suicide bomber killed the head of a local group of Sunni fighters northeast of Baghdad who had turned against al Qaeda insurgents, the leader's brother and a provincial police official said.
Sheik Thaeir Ghadhban al-Karkhi, his 5-year-old niece, a 24-year cousin and a security guard were killed in the blast in Diyala province, where violence has persisted despite drops in other parts of Iraq.
Duraid Mahmoud, the sheik's brother, told The Associated Press he witnessed the attack inside his brother's home. A provincial police official, speaking on condition of anonymity as he was not authorized to release the information, confirmed the attack.
The woman, wearing an explosives belt, entered al-Karkhi's home in the predominantly Sunni town of Kanaan, 13 miles east of Baqouba.
There was no immediate claim of responsibility. But al Qaeda in Iraq has been targeting fellow Sunni Arabs who have taken up arms against the militants and joined the so-called awakening councils like the one al-Karkhi led.
The councils are made up of U.S.-backed former insurgents who have risen up against al Qaeda's brutality and strict Islamic codes of conduct it was trying to impose on local populations.
The U.S. military said it was looking into the incident but did not immediately have any details.
Mahmoud said the bomber had visited the sheik's house on Sunday, claiming that her husband had been kidnapped and asking for help. Mahmoud said his brother told the woman to return Monday.
"She came back this morning and nobody checked her. She had an appointment with the sheik and the guards told her to go and knock on his door," Mahmoud said.
The woman was ushered into the house and blew herself up once she got close to the sheik, he said, adding that the sheik's 5-year-old niece and a security guard also were killed.
One of the men wounded in the attack - the son of a cousin of the sheik - later died at the hospital, according to a hospital official who spoke on condition of anonymity because he was not authorized to speak to the media.
A few hours after the attack, the sheik's coffin was placed in the bed of a pick-up truck and taken for burial, as supporters riding in back waved an Iraqi flag and the banner of the local awakening council.
Dozens of armed men milled about outside al-Karkhi's home, some weeping. A large pool of blood outside the door shimmered in the afternoon sun.
Female suicide bombers have been involved in at least 19 attacks or attempted attacks since the war began, including the grisly bombings of two pet markets in Baghdad that killed nearly 100 people on Feb. 1.
A female suicide bomber last struck in a Shiite neighborhood in Baghdad on Feb. 17, detonating herself after soldiers fired three bullets at her. Causalities were disputed in that attack, with Iraqi officials saying four people were killed, while the U.S. military said only the bomber died.
In southern Iraq, the body of a doctor who was kidnapped on Sunday was found.
Dr. Khalid Nasir al-Miyahi, a neurologist working at a hospital in Basra, was kidnapped by unidentified gunmen, police said. His body was found in a central area of the city.
The police official said the doctor was the only neurologist in Basra.
According to figures from the Iraqi Health Ministry released earlier this year, 618 medical employees, including 132 doctors, as well as medics and other health care workers, have been killed nationwide since 2003. Professionals from many fields have been targeted in Iraq's violence.
Hundreds, possibly thousands, of other medical personnel are believed to have fled to Iraq's northern autonomous Kurdistan region and neighboring countries.