The marine was killed on Thursday during "security and stability operations", the US military said on Friday, without giving further details.
US and Iraqi forces began their offensive on Sunday in a effort to end resistance in Iraq's vast western desert province of al-Anbar, one of the most lawless areas of the country.
They stepped up operations on Thursday, using warplanes and an AC-130 gunship to attack fighters in the town of Haqlaniya, about 240km northwest of Baghdad.
More than 100 suspected fighters have been arrested since the offensive began, and at least three US marines have died.
Separately, the Iraqi interim government said Iraqi forces have captured the leader of an al-Qaida-affiliated group allegedly responsible for carrying out a series of beheadings.
In a statement late on Thursday, the government identified the leader as Muhammad Najam Ibrahim and said he had been arrested by Iraqi National Guardsmen in Baquba, 60km northeast of Baghdad. It gave no date for the arrest.
The government said Ibrahim's operation was linked to Jordanian-born Abu Musab al-Zarqawi, who is said to lead a group in Iraq affiliated to Usama bin Ladin's al-Qaida network.
Officials said Muhammad Najam
Ibrahim beheaded many Iraqis
Ibrahim carried out the beheadings with his brother, the government said.
"The two beheaded a number of citizens in addition to launching attacks against Iraqi security forces," the statement said.
The government said Ibrahim was being interrogated by authorities for more information they hoped would lead to other arrests. Last week, police said they arrested two others in Baquba, including a top aide to al-Zarqawi named Haidar Abu Bawari.