Guardsman Jamie Shadrake was on sentry duty while serving with 1st Battalion Grenadier Guards when he was killed on Friday. His next of kin have been informed. Shadrake had turned 20 on 14 August. He grew up in south Wales and played rugby for Llanharan.
The number of British military deaths since the conflict began in October 2001 now stands at 425.
Guardsman Shadrake, who joined the Army in 2009, died of gunshot wounds when his checkpoint was attacked by insurgents in the Nahr-e Saraj district of Helmand Province. His family described him as a "tremendous son and brother".
They added: "He was proud to be a soldier and died doing a job that he loved. We are all devastated by the loss of Jamie who was such a loving son and brother.
"We are very proud of the fact that Jamie was prepared to do his duty in helping the people of Afghanistan. Jamie lit up any room with his infectious smile. He will be sorely missed by so many who loved him."
His commanding officer, Lt Col James Bowder MBE, said: "Guardsman Shadrake was an extraordinary young man. "He was a talented soldier with a huge amount to offer and would have gone a long way in the Army. Indeed, he was determined to do so, not least in order to impress his elder brother who is a platoon sergeant in the Battalion.
"Our thoughts and prayers are with his parents and siblings at this extremely difficult time."
Defence Secretary Philip Hammond said he was "deeply saddened" to learn of Guardsman Jamie Shadrake's death.
"He was committed to his job, which he conducted with pride and professionalism, and he died for his country, fighting to protect the United Kingdom's national security," he added. "My thoughts and deepest sympathies are with Guardsman Shadrake's family and friends at this most difficult time."