Background: Malaria is endemic in French Guiana, an overseas territory of France on the Guiana Shield. Since 2005, notified malaria cases are decreasing. However, new data show that malaria affects many Brazilian gold miners working illegally in French Guiana, the majority of whom are not counted in official data. In addition, one major concern is the usual practice of improper self-treatment in this mining population, raising fear of the development of antimalarial resistance. This prospective study, conducted in 2015, aimed to estimate the prevalence of Plasmodium spp. in illegal gold miners working in French Guiana. Methods: The recruitment of gold miners was carried out in resting sites along the French Guiana-Suriname border, where they go for supplies, medical care or leisure. After recording agreement, three malaria diagnostic methods were performed : rapid diagnostic test, microscopy and PCR. Results : Among 421 persons recruited in the study, malaria prevalence, detected by nested-PCR, was 22.3% (CI [18.3–26.3], n=94/421) of which 84% were asymptomatic. Conclusions : This significant malaria reservoir in a mobile and illegal population with difficult access to a health care system raises the threat of artemisinin resistance and puts the population of the Guiana Shield at risk of new transmission foci while countries of the region aim at malaria elimination. Even though French legislation may hamper dealing with this population, France must face the reality of malaria in illegal gold miners in order to meet its commitment to malaria elimination.