The soldier, from the Parachute Regiment, died as a result of a suspected improvised explosive device while on a foot patrol near Sangin in Helmand Province on Tuesday afternoon.
His family have been informed.
Another two British servicemen died in separate suspected friendly fire incidents in Helmand this week.
Lance Corporal Michael David Pritchard, 22, of the 4th Regiment Royal Military Police, was killed in Sangin on Sunday.
And a soldier from 3rd Battalion The Rifles, died yesterday from wounds sustained in a firefight near Sangin on Monday.
The MoD said the Royal Military Police were investigating friendly fire as a possible cause of both deaths and said it would not release any more information until the conclusion of inquests.
A total of 243 British troops have died since the start of operations in Afghanistan in 2001, including 106 this year alone.
Colonel Richard Kemp, former commander of British forces in Afghanistan, said friendly fire incidents took place “very frequently indeed” in the chaos of war.
He told BBC Radio 4’s Today programme: “The situation perhaps they (troops) face in Afghanistan, they are fighting in places like Sangin and other towns and villages where there are very tightly-packed compounds, rat-run alleyways, high mud walls, and enemy appearing very, very briefly at short range - it’s kill or be killed.
“You open fire rapidly and sometimes, tragically, you open fire on your own people.”
Col Kemp said he believed the latest incident came after a “sustained and prolonged” fire-fight which resulted in air support being called in.
He said: “Sometimes when air support is delivered very, very closely against the enemy, when friendly forces and enemy forces are close together, that of course can sometimes result in accidents.”
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