This paper describes the design, construction, and evaluation of a simple, battery-operated automated analyzer for on-site determination of reactive soluble phosphorus in polluted water bodies. The analyzer is based on pulsed-flow analysis, implemented with just two solenoid micropumps. Phosphorus is detected using the molybdovanadate photometric method for orthophosphate; detection employing an UV LED is used for increased sensitivity. The whole system is controlled by an ATMega328 microcontroller, including the operation of the flow system, measurement of the absorbance and data processing. Analytical results, as well as other relevant data are sent via wireless internet to a cloud-based internet-of-things (IoT) server, where they can be monitored and retrieved for later processing. Analytical figures of merit were evaluated. Linearity was verified up to a concentration of 6 mg-P L^-1 (R² = 0.996). Precision (s_R (%), n = 8) was in the range of 3%-12% for water samples with phosphorus contents in the 1–5 mg-P L^-1 range. When compared with a reference method, accuracy of the results for those samples (average, n = 8) ranged from 96.2% to 105.8%. Based on these results, its simplicity and low cost, the analyzer was deemed fit for the purpose. The use of cloud storage and IoT technologies provided additional benefits in terms of data availability away from the sampling point.