Harmful cyanobacterial blooms are a major threat to freshwater ecosystems globally weighing down the Good Environmental Status of waters and risk for human health. Biosensors are a highly promising technology that can produce near real-time measurements that could be used by early warning systems safeguarding human and freshwater ecosystem health. We developed a rapid, sensitive, simple and economical test to identify and daily monitor the blooms of potentially toxic cyanobacteria in aquatic environments based on their genetic identity. This innovative solution comes in a colorimetric biosensor format to quickly and efficiently monitor and sustainably manage the contamination risks of potentially harmful cyanobacteria. The biosensor is a genus-specific ribosomal RNA/DNA probe-based assay of potentially toxic genus and is calibrated to equivalent cell numbers. Here we present the field validation tests results performed in a recreative bathing area in south Paris concerning the detection of Microcystis at different sampling points during summer 2017 campaign. The validation was performed by taxonomic cyanobacteria cell counting present in the samples.