A device for optically detecting in transmission nanoparticles moving in a fluid sample includes a light source for emitting a spatially incoherent beam for illuminating the sample; an imaging optical system; and a two-dimensional optical detector. The imaging optical system includes a microscope objective. The two-dimensional optical detector includes a detection plane conjugated with an object focal plane of the microscope objective by said imaging optical system. The two-dimensional optical detector allows a sequence of images of an analysis volume of the sample to be acquired, each image resulting from optical interferences between the illuminating beam incident on the sample and the beams scattered by each of the nanoparticles present in the analysis volume during a preset duration shorter than one millisecond. The device further includes an image processor that allows an average of a sequence of said images to be taken and said average to be subtracted from each image in order to determine, for each nanoparticle of the analysis volume, the amplitude of the scattered beam.