The British soldier, from the 1st Battalion The Royal Anglian Regiment, died in a blast while on foot patrol in the Nad-e Ali area of Helmand Province, yesterday, the Ministry of Defence said.
Next of kin have been informed.
Lieutenant Colonel David Wakefield, Spokesman for Task Force Helmand, said of the soldier: "His dauntless courage and his sacrifice will not be forgotten."
His death takes the number of British service personnel killed since the start of operations in Afghanistan in 2001 to 246.
The four US troops were the first American military casualties in Afghanistan in 2010. They were killed in an improvised explosive device strike, NATO-led forces said in a statement.
Meanwhile, the Taleban today said a series of explosions in Panjwai district of southern Kandahar killed several foreign soldiers yesterday.
Last year, the number of US service personnel to die in Afghanistan was more than twice that in 2008.