The airman, whose next of kin have been informed, was shot dead while on a routine patrol of the Western Dashte area, said the Ministry of Defence.
British forces spokesman Lt Col Gordon Mackenzie said "Our thoughts and prayers are with his family and friends at this difficult time."
British military deaths in Afghanistan since 2001 now stand at 398.
The airman had been on a partnered patrol and so would have been interacting with civilians while accompanying an Afghan officer.
His death comes after Lance Corporal Gajbahadur Gurung from 1st Battalion The Yorkshire Regiment was also killed by gunfire, in Helmand's Khar Nikah area on 27 January.
Some 500 of around 9,500 British servicemen and women in Afghanistan are due to be brought home in 2012 and Prime Minister David Cameron has indicated that more will leave in 2013.
He has said he wants British forces to end combat operations in the country by 2014.
A Royal Air Force Regiment airman has been killed during an insurgent attack in the Nad-e Ali district of Helmand.