The vehicle Mr Gilkey, 50, and Mr Tamanna, 38, were travelling in was struck by shellfire near the town of Marjah, NPR said.
Mr Tamanna was a photographer and journalist in Afghanistan, as well as a translator.
A veteran US photojournalist and a translator have been killed in Helmand province in southern Afghanistan.
David Gilkey of National Public Radio (NPR) and Zabihullah Tamanna were travelling with the Afghan army when they came under fire and their vehicle was hit by a shell, NPR said.
The attack also killed the driver of the vehicle, an Afghan soldier.
Two other NPR employees travelling with the pair were unharmed, the US public radio network said in a statement.