The spokesman added that the serviceman was killed at 0645 local time (0345 BST) in the valley, which is around five miles (8km) from the town of Gereshk.
Marines from 40 Commando took on responsibility for Battle Group North in Helmand Province from last September until this April, where they operated from spartan forward operating bases in the Gereshk and Sangin valleys, often taking part in fierce gun battles with the Taliban, and playing a key role in the recapture of the town of Musa Qala.