Phenobarbital, also known as Luminal, is a barbituric acid derivative that possesses an intense hypnotic and sedative effect. It also has a secondary important anti-convulsant effect. The main purpose of this research consisted of the development and optimization of a new, simple, rapid, and accurate spectrophotometric method in the visible range (VIS) for quantitative analysis of Phenobarbital from various different pharmaceutical samples. Beta-naphthylamine from 0.1 % alcoholic solution underwent a complete diazotization reaction in the presence of sodium nitrite NaNO₂, 5%, and hydrochloric acid HCl, 15%-20%, for half an hour in cold conditions (0-5°C). The diazonium salt of beta-naphthylamine formed was then quantitatively coupled with Phenobarbital from a 5% sodium hydroxide NaOH solution, which led to the synthesis of a bright red-orange dye providing a prominent maximum absorption wavelength at ʎ = 487 nm. This azo dye was formed in equivalent proportions to the Phenobarbital from the sample and was spectrophotometrically determined. Through spectrophotometric determination of the bright red-orange azo dye at ʎ = 487 nm, the content expressed in milligrams of pure Phenobarbital from the tablets was effectively calculated. The pure Phenobarbital content calculated was found to be 97.104 mg, very close to the official amount of 100 mg stated by the pharmaceutical company. This pure amount found corresponded to a 97.104 mg % content. The average percentage deviation from the official reference value (100 mg) of the 97.104 mg Phenobarbital content calculated was only (+) 2.896 %, located below the maximum allowed limit of ± 5% imposed by the European Pharmacopeia and by the Romanian Pharmacopoeia's official rules. Finally, the statistical validation procedure consisted of a linearity analysis and detection limit (LOD) and quantitation limit (LOQ) calculations, as the first stages. The stability of the prepared solutions, the system's precision, the intra-day and inter-day precision of the method, and the accuracy of the method were also within the normal range of values.