The Royal Marine, from 40 Commando, and the female soldier, from 3 Medical Regiment, were on patrol in the Nahr-e Saraj district of Helmand province.
Their families have been informed.
The pair are thought to have been killed by insurgents, though other reports claim the incident was a “green on blue” attack, where coalition troops are killed by their Afghan allies.
Major Laurence Roche, a spokesman for Task Force Helmand, said: “I am extremely sorry to announce the deaths of a Royal Marine from 40 Commando and a soldier from 3 Medical Regiment serving with Task Force Helmand.
“This is dreadful news for all of us serving in Afghanistan. Our sincere condolences go to their families, friends and colleagues at this time of grief,” he said.
A total of 435 British military personnel have been killed in Afghanistan since operations began in 2001.