The Marine, who was serving as part of Combined Force Sangin, was shot while on a foot patrol in the Sangin district of Helmand province.
Spokesman for Task Force Helmand, Lieutenant Colonel James Carr-Smith, said:
"It is with deep sadness I must inform you that a Marine from 40 Commando Royal Marines was killed in Sangin this evening.
"He was part of a foot patrol, together with his fellow Marines and Afghan National Army partners, when he was killed by small arms fire. He died in the course of his duty.
"He was a brave and courageous Marine who made the ultimate sacrifice. He will be greatly missed and his actions will not be forgotten. We will remember him."
Next of kin have been informed and have asked for a period of grace before further details are released.