Marine Steven Birdsall, of 40 Commando Royal Marines, was shot by insurgents in Sangin in Helmand Province on Sunday.
He was flown back to Queen Elizabeth Hospital in Birmingham but died from his wounds on Monday afternoon.
The 20-year-old, who lived in Warrington, Cheshire, with his mother, father and younger sister, was described by his commanding officer as "the perfect marine".
His parents said in a statement: "There are no words that could ever express the heartache of losing our beautiful son Steven who was always so selfless, brave and fearless."
He joined the Royal Marines in 2007, aged 18, and after training deployed to Afghanistan in April.
The young marine was shot while he guarded a team of Royal Engineers reinforcing the defences at one of the British checkpoints in Sangin.
Lieutenant Colonel Paul James, commanding officer of 40 Commando Group, said the marine's "gallantry, selflessness and determination" impressed everyone who knew him.
He said: "He possessed a sharp mind and a big and generous heart. He loved his family, his friends and his fellow marines, and they adored him in return. He was a consummate professional – forever focused, very proud and utterly dependable, yet always cheerful and magnanimous."
Defence Secretary Liam Fox said the marine had "given his life helping to deliver real progress in Afghanistan, which is at the heart of our efforts to protect security in the UK."
Two British soldiers from 1st Battalion The Duke of Lancaster's Regiment were shot dead in separate incidents in the Nad-e Ali district of Helmand on Tuesday. Corporal Taniela Tolevu Rogoiruwai, 32, and Kingsman Ponipate Tagitaginimoce, 29, both from Fiji, were hailed by comrades for "excelling" in their roles.
The UK death toll since operations in Afghanistan began in 2001 now stands at 298.
David Cameron rejected calls in the Commons yesterday to withdraw UK troops from Afghanistan immediately, while acknowledging the war was "costing us dearly".
The Prime Minister was told that bringing about 10,000 British service personnel home from the country would save more than 7m a day.
But Mr Cameron said leaving Afghanistan would increase the risk of terrorist attacks as well as letting down international allies and the Afghan people.