The Marine, assigned to Regimental Combat Team 5, died from injuries sustained during combat on Sunday, the military said in a brief announcement. The Marine's name was being withheld pending notification of next of kin.
A death on Saturday that had not been reported by the military in Iraq was later reported by the Pentagon in Washington, driving October's toll to 100.
October already was the fourth deadliest month for American troops since the war began in March 2003. There were 107 service members slain in January 2005; at least 135 in April 2004, and 137 in November 2004.