Because trace amounts of nitrite would cause adverse effects on human health, the development of low-cost, portable, and easy-to-use tools is very significant for tracking and evaluating nitrite contamination in water. Herein, we described a smartphone-based electrochemical system for rapid quantification and mapping of nitrite contamination. The system contained a smartphone, a hand-held detector, and a modified screen-printed carbon electrode (SPCE). With a home-made Android application, the smartphone was used to control the detector, display the detection results, and link the system to the Cloud. The hand-held detector (∼42 g) was designed to apply the excitation signals on the modified SPCE and transmit the response data to the smartphone via Bluetooth. The SPCE was used as a sensor for nitrite sensing, on which gold nanoparticles, NiO nanoparticles, and reduced graphene oxide were in-situ electrochemically deposited by the system. The smartphone-based system showed excellent analytical performance for the detection of nitrite with a low detection limit of 0.2 μM in a linear range of 1-500 μM. Furthermore, a nitrite contamination map was constructed by measuring the water samples at different locations in Hainan Province, thus showing great potential of this system for water quality monitoring, especially in low-resource countries.