The US military said in a statement that the Marine had died yesterday in combat but did not provide any more details.
US President George W Bush is facing mounting pressure over his Iraq policy ahead of November 7 mid-term congressional elections in which his Republicans could lose control of both houses.
Before the latest death, October was already the deadliest month since January 2005 when 107 US troops were killed.
The highest monthly toll was in November 2004 when 137 deaths were recorded.
The total number of US deaths in Iraq since the US-led invasion in March 2003 is 2,813. Iraqi security forces have suffered even greater casualties.
The US commander in Iraq said this week 300 Iraqi police and soldiers were killed during the holy month of Ramadan which ended in the past week.
Hundreds of civilians are killed every week in sectarian attacks by militias and insurgents.
29 labourers killed in Iraq
A bomb has ripped through a crowd of labourers waiting for work in the restive Sadr City district of Bagdad, killing at least 29 people and wounding 59 more, according to the Iraqi interior ministry.
Ministry spokesman Brigadier General Abdel-Karim Khalaf said this death toll was provisional and expected to rise as the wounded were ferried to hospital.
The bomb was hidden in a rubbish bin and triggered when casual labourers had gathered at a road side waiting for employers, according to another security source.
There was no initial word on who might have planted the device, but suspicion will fall on Sunni extremists seeking to provoke sectarian violence.
Sadr City is a sprawling Shiite slum in eastern Baghdad and a bastion of the Mahdi Army militia.
The district is currently surrounded by a cordon of US troops searching for a kidnapped American soldier.
Gunmen kill 17 Iraqi police
The attack has come hours after gunmen have killed 17 Iraqi police trainers and two translators on a bus outside the southern city of Basra.
An Iraqi officer says the 19 men were on their way home from the British-run police academy in the town of Shuaiba.
The murders are a huge setback for British efforts to train local security forces to take control of an area rife with illegal militia groups.
The killings came as US troops fought off an insurgent ambush in northern Iraq, killing 17 rebels.
Despite the ongoing violence, Iraq's government has urged the United States to grant it more control over its own armed forces.
Authorities say they were killed and their bodies were taken back to Shuaiba and scattered around the town.
The two translators work with the instructors and the British army and police," they said, speaking on condition of anonymity.
"The Iraqi police (IPS) dealt with the situation from its first report and continue to investigate," said British military spokesman Major Charlie Burbridge.
"We are poised to support with whatever the IPS request within our capabilities. We are taking the incident extremely seriously indeed," he said.
The murders will be seen as a huge setback for British efforts to pacify southern Iraq, which hinges on training Iraqi security forces to enable them to take over responsibility for an area rife with illegal militia groups.
Although the area around Basra, which is largely Shi'ite, has been spared the worst of the sectarian violence that has torn apart life in central Iraq, the region is home to well-armed political, tribal and criminal factions.
The British government is under intense domestic political pressure over its strategy, with the former head of the country's armed forces branding the attempt to fight on two fronts - in Iraq and Afghanistan - as "cuckoo".
Britain has 7,200 troops in southern Iraq.
Reports suggest their commanders would like to start pulling them out as early as February next year, although Prime Minister Tony Blair insists they will stay "until the job is done".