The invention relates to a method for counting particles, particularly blood cells, in a sample, using a lensless optical imaging device. The sample is arranged between a light source and an image sensor. The sample is illuminated by a light source and an image is acquired by the image sensor, said image sensor being exposed to a light wave called an exposition wave. A digital propagation operator is applied to the acquired image so as to obtain a complex amplitude of the exposition wave according to a surface facing the image sensor. An image, called a reconstructed image, is formed from the modulus and/or the phase of said complex amplitude, on which image the particles to be counted appear in the form of regions of interest. The method then comprises a step of selecting the regions of interest corresponding to the particles to be counted.