Dr. Kent Brantly, medical director at one of the country's two treatment centers run by the organization, recognized his own symptoms and confined himself to an isolation ward.
It was not immediately clear how he caught Ebola. The relief group's Melissa Strickland said that he had followed strict safety protocols when treating patients.
"It's too early to try to explain it. We will have an intensive and thorough investigation," she said.
Across Guinea, Liberia and Sierra Leone, at least 660 people have died from the illness, according to the World Health Organisation, as poor, ill-equipped African governments have struggled to cope with the virus.
Ebola kills up to 90 percent of those who catch it, although the fatality rate of the current outbreak is lower at around 60 percent. Highly contagious, patients suffer from vomiting and diarrhea as well as internal and external bleeding.