In the present work, an automated procedure for photometric ammonium determination in rainwater using a multicommuted flow analysis approach is proposed. A homemade LED based photometer was designed to couple a flow cell with a path length of 100 mm in order to improve sensitivity. After establishing the adequate operational conditions, the proposed procedure was applied for the determination of ammonium in rainwater samples. The assessment of accuracy, by applying the paired t-test between these results and the results obtained using a reference method, showed that there is no significant difference at a 95% confidence level. Other useful features such as a linear response ranging from 0.05 to 1.20 mg L⁻¹ NH₄⁺; a detection limit of 19 μg L⁻¹ NH₄⁺; a blank relative standard deviation (n = 15) of 2.9%; a reagent consumption of 1.5 mg sodium salicylate and 0.15 mg sodium nitroprusside; a waste generation of 3.1 mL per determination; and a sampling throughput of 38 determinations per hour were also achieved.