It was one of the heaviest daily tolls for US forces in the violent region.
"This was a car bomb not too far from Fallujah," said Lieutenant Lyle Gilbert.
In a statement, the marines adjusted an earlier updated casualty count from nine dead and nine wounded, without offering any explanation for the change.
"The total number of marines killed in action decreases to eight, wounded in action increases to 10," it said.
The latest deaths brought to 1,112 the number of US military personnel killed in Iraq since the US-led invasion in March 2003, according to a Pentagon tally.