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.

The logo, called Harmonized Chequered Emblem, replaces the first choice which was thrown out last year after the designer was accused of plagiarism.
The designer denied stealing the idea.
Organisers said the new design used traditional Japanese colours and patterns to represent the intercultural themes of the Games.
"It incorporates the message of 'unity in diversity'," they said, and the idea that the Games "seek to promote diversity as a platform to connect the world".
When designer Asao Tokolo found out he had won the re-opened contest he said "my mind has gone blank".
"I put a lot of time and effort into this design as though it was my own child."
The first design was rejected after Belgian artist Olivier Debie alleged it copied his design for a theatre logo.
The Games organising committee never agreed to the allegation of plagiarism but said there were too many doubts over the emblem for it to be used.
The logo dispute came shortly after Tokyo decided to scrap the designs for the main Olympic stadium because of spiralling construction costs.
A cheaper design, by Japanese architect Kengo Kuma, was chosen in December last year.

The average mileage for four-wheeled vehicles stood at 7,900 miles (12,700km) in 2013, official figures showed.
This represents a 14% fall from a figure of 9,200 miles in 2002, the first year for which data is available.
Average annual mileage has dipped steadily in the intervening period, with mileage rising only in 2007.
The average figure for 2013 fell from the previous year even though the distance travelled when commuting to and from work increased from 2,500 miles in 2012 to 2,800 miles last year.
Business mileage dropped from 800 miles in 2012 to 700 miles in 2013. In 2002, this figure stood at 1,300 miles.
RAC Foundation director Professor Stephen Glaister said: "While it is important to see whether individuals are driving more or less than before, official figures show overall traffic volume is on the rise again following the end of the recession.
"With the population expected to increase by 10 million over the next two decades congestion is going to get worse, not better."
AA president Edmund King said: "The weather in 2013, spikes in fuel prices and an increase in online shopping have all contributed to the drop in car use.
"Although the coldest March in 50 years will have contributed to the fall in petrol sales in 2013, by far the biggest cause over the first quarter was the third 8p to 10p-a-litre price swing in 12 months."
The figures also showed that people in England travelled 6,592 miles on average in all modes of transport last year, including walking and travelling as a passenger in a car or van.
This figure is lower than the headline 7,900-mile figure for drivers, which applies to each vehicle, many of which are driven by more than one person.
Each person on average walked 184 miles and travelled 288 miles by bus.
The largest users of cars and vans were those living in rural areas, who walked only 97 miles on average last year.
Those living in London walked the most, averaging 215 miles, while those in the West Midlands walked the least, clocking up only 161 miles.
Car and van occupancy has not changed in recent years with the 2013 average being 1.57 people per journey, the same as in 2012 and 2011.
A Royal Air Force Regiment airman has been killed during an insurgent attack in the Nad-e Ali district of Helmand.
[NEXT_CONCEPT]
Japan's Olympic organisers have unveiled the new official logos of the 2020 Tokyo Olympic and Paralympic Games.
[NEXT_CONCEPT]
Drivers in England are clocking up the lowest number of miles on average since records began 12 years ago.