Authorities arrested four people who were at the same nightclub as the two victims on Sunday, but have not yet established a firm link to the murders.
Prosecutor Thierry Rajaona Lauret, who is running the investigation, said a fifth suspect — a French national living on the resort island of Sainte Marie — was arrested on Tuesday morning.
There is “no doubt” the two volunteers were murdered, police said. “We just need to determine a motive.”
The bodies of the two French citizens, a man and a woman aged in their 20s, were found Sunday on a beach of Sainte Marie with head injuries.
They had been volunteering at Cetamada, a local environmental non-profit organisation working to protect ocean mammals.
According to authorities on Sainte Marie, an island famous for eco-tourism, it is the first time French citizens have been murdered there.
In 2012, a French couple was found dead on a beach in southern Madagascar.
A year later, two Europeans were attacked and killed by a mob on the northwest island of Nosy Be, suspected of involvement in the death of an eight-year-old local boy.