The men were killed during an operation against illegal gold miners in the western Dorlin region of French Guiana, which is located along the northeast coast of South America.
A team of around a dozen gendarmes and at least 18 soldiers were dropped into the area after a helicopter providing security for a legal mining operation in the remote region came under fire early Wednesday.
"They fell into an ambush," said the local prefecture.
The lives of the two gendarmes were not thought to be in danger, said Colonel Didier Laumont, commander of the local gendarmerie.
France's minister for the overseas territories Victorin Lurel was due to fly in Thursday to attend the dead soldiers' funerals and visit the wounded, his ministry said.
The Dorlin region has for years been the site of illegal gold mining, which has occasionally led to deadly violence.
In January, at least five people were killed during a shootout between rival gangs, just hours ahead of a visit by then president Nicolas Sarkozy.
The government has been trying to stamp out the illegal industry, which causes serious ecological damage including mercury-polluted waters.