In a statement on Friday, the US military said "one Marine assigned to the I Marine Expeditionary Force was killed in action on Thursday, 16 December while conducting security and stabilisation operations in the al-Anbar province".
No further details were provided.
Al-Anbar, a vast province covering much of western Iraq, includes the cities of Falluja and Ramadi, where fighting between US forces and Iraqi fighters have been near constant for the past six weeks.
Aljazeera has also learned that three Iraqi national guardsmen, who were on their way home, were killed in an ambush in the Abu Ghraib neighbourhood west of Baghdad.
In a separate incident, clashes erupted between armed fighters and US forces in the al-Jamaa district west of the capital.
In the town of Baiji in northern Iraq a US tank was destroyed by an explosive device on the highway.
Also in the northern Iraqi town of Kirkuk, a rocket slammed into a Kurdish refugee housing compound, killing three refugees and wounding one, police Brigadier General Sarhad Qadir said.