The American was killed on Friday "as a result of an IED strike in southern Afghanistan," NATO's International Security Assistance Force (ISAF) said in a statement, referring to an improvised explosive device.
IEDs, which are cheap and easy to make and difficult to detect, account for a growing number of troop deaths in Afghanistan, as the war against Taliban-led insurgents escalates.
So far this year 496 foreign troops have died in the country, according to an AFP count based on a tally kept by the independent website icasualties. Of those deaths, 306 were American.
US President Barack Obama has ordered an extra 30,000 troops to Afghanistan, and NATO countries have pledged around 7,000 more.
They are expected to deploy by mid- to late-2010 and analysts say more foreign troops will inevitably mean more deaths.
There are currently more than 100,000 foreign troops battling a resurgent Taliban under US and NATO command.