Another US soldier and two other Afghan National Army soldiers were also wounded in the small-arms and rocket-propelled grenade attack, which occurred Friday while they were conducting combat operations north of the city of Kandahar, the military said in a statement on Saturday.
The three wounded soldiers were taken to a nearby forward operating base for treatment, according to a statement from the main US base at Bagram, north of the capital Kabul.
There was no immediate word on their condition.
The American's death brought the number of US service members killed in and around Afghanistan since the 2001 campaign to oust the hard-line Taliban from power to 198.
Attacks against US soldiers have steadily risen in the past few weeks.
On 27 September, two US troops were killed in separate attacks in southern and eastern Afghanistan, just over a week after the country held landmark elections.