The bomb exploded near a marine amphibious assault vehicle as it was travelling south of Haditha, a town on the Euphrates river about 200km northwest of Baghdad.
A civilian translator was also killed, and one Marine was wounded.
It is the second major attack against marines in the area in the past three days and one of the deadliest attacks against US forces since the beginning of the war.
On Monday, six marines were killed in clashes in Haditha, and a seventh was killed by a car bomb blast in Hyit, southeast of the town.
The western Anbar governorate of Iraq has been one of the deadliest regions for US forces since they invaded Iraq in March 2003.
The towns of Falluja and Ramadi are in Anbar.
In December, 22 people were killed including 14 US servicemen when a bomber blew himself up inside a mess hall at a military base in the northern city of Mosul.
That was the deadliest attack on a US installation since the war.
At least 1820 US troops have died in Iraq since the beginning of the war.
In the past month, more than 60 have died, many of them in Anbar. US forces have launched two major offensives in the area since May to try to crush insurgents.
US journalist killed
The US embassy in Iraq earlier said that an American freelance journalist had been found shot dead in the southern Iraqi city of Basra.
Police on Wednesday said Steven Vincent was shot several times after he and his Iraqi translator were abducted at gunpoint hours earlier.
"I can confirm to you that officials in Basra have recovered the body of journalist Steven Vincent," said embassy spokesman Pete Mitchell.
"The US embassy is working with British military and local Iraqi officials in Basra to determine who is responsible for the death of this journalist. Our condolences go out to the family."
The embassy did not give the cause of death.
However, Iraqi police in Basra said Vincent was abducted along with his female translator at gunpoint on Tuesday evening. The translator, Nour Weidi, was seriously wounded.
Vincent and the translator were taken by five men in a police car as they left a currency exchange shop, police Lieutenant Colonel Karim al-Zaidi said.
Vincent's body was found on the side of the highway south of Basra later. He had been shot in the head and multiple times in his body, al-Zaidi said.
Police said Vincent, a writer who had been living in New York, had been staying in Basra for several months working on a book.
In an article published on 31 July in The New York Times, Vincent wrote that Basra's police force had been heavily infiltrated by members of Shia political groups, including those loyal to cleric Muqtada al-Sadr.
Vincent quoted an unidentified Iraqi police lieutenant as saying that some police were behind many of the assassinations of former Baath Party members that had taken place in Basra.
"He told me that there is even a sort of 'death car': a white Toyota Mark II that glides through the city streets, carrying off-duty police officers in the pay of extremist religious groups to their next assignment," he wrote.
UK army criticised
Vincent was also critical of the British military, which is responsible for security in Basra, for turning a blind eye to abuses of power by Shia extremists in the city.
He was the author of In the Red Zone: A Journey into the Soul of Iraq, a recently published book that was an account of life in post-Saddam Iraq.
Vincent's website describes him as a freelance investigative journalist and an art critic whose work had appeared in major newspapers and magazines including the Wall Street Journal, Harper's and the Christian Science Monitor.
He had been working on a book about the history of Basra.
According to the New York-based Committee to Protect Journalists, as of 28 June, 45 journalists and 20 media support workers have been killed while covering the war in Iraq since March 2003.