The Water Framework Directive (WFD) requires European Union Member States to monitor the ecological quality of their water bodies through the responses of key biological compartments to anthropogenic alteration in the aquatic environment. French Guiana is a French overseas department located in the Amazonian part of South America. Despite its geographical remoteness, this department is a European territory like all the other French overseas departments and is therefore subjected to the requirements of the WFD. Previous work over the past decade, especially on benthic diatoms, have led to the development of several new biological indices specifically adapted to the biogeographic conditions (geological substrates, climate, local fauna and flora) of each of these overseas territories. However, work has progressed at a slower rate on the diatoms of French Guiana, due to the particular natural conditions that lead to very specific and still largely unknown diatomic communities. Since no convenient local system was available, an evaluation system based on a local adaptation of a generalist diatom index (SPI) has been previously set up to assess the ecological state of French Guiana's watercourses, despite obvious weaknesses linked to the biogeographical mismatch of the ecological profiles of many taxa. The increase in the quantity of data gathered through Guyanese monitoring networks over the past 9 years, as well as the progress made on diatom taxonomy, allowed the development of the new French Guiana Diatomic Index (FGDI) presented in this paper. A methodology adapted to the Guyanese context has been developed based on (i) a reduction in the number of taxa taken into account in diatom communities and on (ii) the search for alert taxa in a multimetric approach. The FGDI allows a relevant and consistent ecological status evaluation of French Guiana's streams based on benthic diatoms following the WFD framework and, due to its multimetric construction, it also allows a comprehensive ecological diagnosis which will be of great benefit for water managers.