The deadly Ebola virus was contracted by a female French volunteer working for Doctors Without Borders (MSF) while on assignment in Liberia, the charity said Wednesday.
It marks the first time MSF has announced that a French national has been diagnosed with the disease.
The woman was placed in isolation on Tuesday "after she developed a fever", MSF said in a statement. Tests later confirmed she had been contaminated with the tropical virus that has been ravaging west Africa.
The staff member, who was working in Monrovia, will soon be transferred to a specialized treatment centre in France, the organization said.
"MSF applies very strict protocols of protection for its staff -- before, during and after their time in a country affected by the current Ebola outbreak," said Brice de le Vingne, MSF Director of Operations.
"This dramatically reduces the risk of transmission of the disease. However, the risk is part of such an intervention, and sadly our teams are not spared."
For privacy reasons, no further details were given about the affected volunteer.
MSF says it currently has more than 2,000 staff members working to combat Ebola in west Africa, including some 200 international workers.
The international community is scrambling to respond to the current Ebola outbreak, the worst on record. The virus has killed more than 2,400 people in Liberia, Guinea, Sierra Leone and Nigeria this year.
The United Nations Security Council is to hold an emergency session on Thursday to discuss ramping up the global aid response to the crisis.
US President Barack Obama on Tuesday announced the United States would send 3,000 military personnel to west Africa to tackle the outbreak, which he said was "spiraling out of control".