This paper describes the design, testing and validation of a simple and inexpensive spectrophotometric method for total Se analysis in aqueous solution based on the azo coupling reaction of amines. In this reaction, selenite ions react with hydroxylamine hydrochloride (NH2OH.HCl), which in turn diazotizes p-Nitroaniline and is coupled with N-1-naphthyl-ethylenediamine dihydrochloride (NEDA) to form a red coloured azo compound. This azo compound has a maximum absorption peak at 545 nm, an apparent molar absorption of 2.85±104 l mol−1 cm−1 and it follows the Beer’s law in the range from 0.01 to 2.50 mg/l with R2 = 0.9998. Interferences between the azo reaction and non-targeted ions often present in environmental samples (i.e., NO3−, CO3−, Cl− or metals) were investigated and specific protocols to overcome them were implemented. The advantages of the new method for low levels of Se in aqueous solutions are its high sensitivity and reproducibility, and the fact that measurements are simple, rapid and low-cost.