The majority of existing advanced automated cell counter instruments used in laboratory settings are complex, expensive, and bulky. As a result, applications of these instruments have been limited to such laboratories. Meanwhile, in many rural areas and developing countries, clinical laboratories equipped with optical microscopy, hematology analyzers or commercial automated particle counters may not be readily accessible to everyone. However, in the same regions, the number of cell-phone users are rapidly increasing, suggesting a need to develop an easy-to-use and portable smart-phone based cell counting technology that can be leveraged in resource-limited areas. To this end, we present an automated, portable and easy-to-use smartphone-based particle counting platform designed to detect particles in a sample solution. This novel pump-less, flow-based portable technology utilizes a small lens attached to a smartphone camera to magnify particles passing through a microfluidic channel, and record a video using a smartphone camera application. The captured video is transmitted to a local computer to be processed through a custom-developed algorithm to perform particle counting. Using a total of 30 different test samples, we have shown that our technology can detect and count polystyrene beads as small as 16.2 μm with ~100% accuracy. We evaluated the performance of our new technology using different size particles and showed that the results are comparable to the tedious, manual-microscope based counting method. Although, we benchmark the performance of the platform using polystyrene beads, we emphasize the wide applicability of the platform to a wide range of biological, biomedical, and industrial applications.