With the increasing capabilities and ubiquity of smartphones and their associated digital cameras, this study presents a smartphone integrated optoelectrowetting (SiOEW) device as a simple, portable tool capable of on-chip water sample preparation and microscopic detection of the target cells in water samples, which significantly reduce the detection time and the labor cost required for water quality monitoring. A commercially available smartphone is used as a low-intensity portable light source to perform optoelectrowetting (OEW)-based microfluidic operations such as droplet transportation, merging, mixing, and immobilization on a hydrophobic detection zone. Furthermore, a built-in smartphone camera allows on-chip microscopic detection of water quality with a 45× magnification. We have experimentally demonstrated that the SiOEW platform is able not only to automate the sample processing of marine water including the target algae cells (Amphiprora sp.) and staining reagents fluorescein diacetate (FDA) and 5-chloromethylfluorescein diacetate (CMFDA), but also detect the fluorescence signals emitted from the target cells in water samples and count their populations. Using the smartphone, the collected information (e.g. the location of the water sample collected and the time it was detected, the number of the target cells, etc.) can be rapidly and wirelessly shared with a central host such as an environmental regulation agency for real-time monitoring and further management of water quality.